Resolve compiler warning about _tree.
Resolve compiler warning about _tree.

Add unused routines in order to suppress the error message:

include/RooTreeData.h:33:10: error: private field '_tree' is not used [-Werror,-Wunused-private-field]
  TTree *_tree ;           // TTree holding the data points
         ^

This re-apply commit f4d184e64190d7ad67075d052fb548d40e8a1de5 (May 10).

krafczyk/root,abhinavmoudgil95/root,root-mirror/root,esakellari/root,buuck/root,thomaskeck/root,abhinavmoudgil95/root,gbitzes/root,gbitzes/root,bbockelm/root,omazapa/root,karies/root,Y--/root,omazapa/root,vukasinmilosevic/root,dfunke/root,BerserkerTroll/root,lgiommi/root,mattkretz/root,Y--/root,root-mirror/root,arch1tect0r/root,dfunke/root,Duraznos/root,gbitzes/root,simonpf/root,root-mirror/root,bbockelm/root,BerserkerTroll/root,thomaskeck/root,dfunke/root,omazapa/root,smarinac/root,perovic/root,CristinaCristescu/root,root-mirror/root,0x0all/ROOT,sirinath/root,CristinaCristescu/root,thomaskeck/root,perovic/root,sirinath/root,jrtomps/root,evgeny-boger/root,beniz/root,gganis/root,pspe/root,beniz/root,nilqed/root,krafczyk/root,esakellari/my_root_for_test,sbinet/cxx-root,davidlt/root,gganis/root,root-mirror/root,omazapa/root,perovic/root,zzxuanyuan/root,agarciamontoro/root,omazapa/root-old,bbockelm/root,zzxuanyuan/root-compressor-dummy,olifre/root,CristinaCristescu/root,mhuwiler/rootauto,zzxuanyuan/root,davidlt/root,georgtroska/root,sbinet/cxx-root,pspe/root,esakellari/my_root_for_test,veprbl/root,smarinac/root,zzxuanyuan/root,veprbl/root,agarciamontoro/root,omazapa/root,mattkretz/root,vukasinmilosevic/root,veprbl/root,esakellari/my_root_for_test,arch1tect0r/root,mkret2/root,buuck/root,abhinavmoudgil95/root,omazapa/root-old,arch1tect0r/root,thomaskeck/root,davidlt/root,lgiommi/root,beniz/root,lgiommi/root,cxx-hep/root-cern,zzxuanyuan/root,arch1tect0r/root,davidlt/root,mkret2/root,satyarth934/root,veprbl/root,vukasinmilosevic/root,krafczyk/root,simonpf/root,omazapa/root-old,CristinaCristescu/root,buuck/root,buuck/root,olifre/root,olifre/root,Duraznos/root,simonpf/root,bbockelm/root,mkret2/root,Duraznos/root,davidlt/root,esakellari/root,pspe/root,buuck/root,cxx-hep/root-cern,sawenzel/root,mattkretz/root,CristinaCristescu/root,arch1tect0r/root,0x0all/ROOT,mattkretz/root,zzxuanyuan/root-compressor-dummy,zzxuanyuan/root,bbockelm/root,jrtomps/root,0x0all/ROOT,0x0all/ROOT,mhuwiler/rootauto,sbinet/cxx-root,Y--/root,Y--/root,cxx-hep/root-cern,esakellari/my_root_for_test,georgtroska/root,Y--/root,veprbl/root,gbitzes/root,mkret2/root,dfunke/root,Y--/root,agarciamontoro/root,evgeny-boger/root,vukasinmilosevic/root,zzxuanyuan/root-compressor-dummy,nilqed/root,beniz/root,BerserkerTroll/root,dfunke/root,veprbl/root,omazapa/root,sawenzel/root,veprbl/root,sbinet/cxx-root,sirinath/root,smarinac/root,sawenzel/root,olifre/root,veprbl/root,mattkretz/root,mkret2/root,Duraznos/root,mkret2/root,jrtomps/root,evgeny-boger/root,Y--/root,karies/root,CristinaCristescu/root,satyarth934/root,dfunke/root,BerserkerTroll/root,vukasinmilosevic/root,gganis/root,olifre/root,buuck/root,cxx-hep/root-cern,buuck/root,beniz/root,arch1tect0r/root,vukasinmilosevic/root,mhuwiler/rootauto,mkret2/root,davidlt/root,zzxuanyuan/root-compressor-dummy,lgiommi/root,BerserkerTroll/root,nilqed/root,olifre/root,lgiommi/root,evgeny-boger/root,beniz/root,esakellari/my_root_for_test,root-mirror/root,smarinac/root,abhinavmoudgil95/root,simonpf/root,smarinac/root,beniz/root,arch1tect0r/root,omazapa/root,sawenzel/root,nilqed/root,abhinavmoudgil95/root,pspe/root,satyarth934/root,nilqed/root,buuck/root,mattkretz/root,dfunke/root,jrtomps/root,zzxuanyuan/root-compressor-dummy,karies/root,vukasinmilosevic/root,smarinac/root,mkret2/root,Duraznos/root,georgtroska/root,nilqed/root,nilqed/root,BerserkerTroll/root,jrtomps/root,perovic/root,jrtomps/root,thomaskeck/root,omazapa/root,CristinaCristescu/root,olifre/root,mattkretz/root,mhuwiler/rootauto,pspe/root,esakellari/root,mhuwiler/rootauto,satyarth934/root,esakellari/root,bbockelm/root,evgeny-boger/root,abhinavmoudgil95/root,sawenzel/root,thomaskeck/root,satyarth934/root,smarinac/root,lgiommi/root,0x0all/ROOT,CristinaCristescu/root,evgeny-boger/root,sawenzel/root,vukasinmilosevic/root,beniz/root,Duraznos/root,pspe/root,zzxuanyuan/root-compressor-dummy,perovic/root,Y--/root,gbitzes/root,esakellari/root,mhuwiler/rootauto,pspe/root,abhinavmoudgil95/root,Duraznos/root,karies/root,mkret2/root,CristinaCristescu/root,mattkretz/root,mhuwiler/rootauto,satyarth934/root,sirinath/root,mattkretz/root,zzxuanyuan/root,omazapa/root-old,0x0all/ROOT,jrtomps/root,smarinac/root,veprbl/root,krafczyk/root,esakellari/root,esakellari/root,evgeny-boger/root,evgeny-boger/root,krafczyk/root,omazapa/root,satyarth934/root,sbinet/cxx-root,arch1tect0r/root,beniz/root,agarciamontoro/root,sbinet/cxx-root,krafczyk/root,krafczyk/root,sawenzel/root,evgeny-boger/root,abhinavmoudgil95/root,esakellari/root,mhuwiler/rootauto,karies/root,omazapa/root-old,veprbl/root,Duraznos/root,vukasinmilosevic/root,thomaskeck/root,davidlt/root,abhinavmoudgil95/root,jrtomps/root,root-mirror/root,perovic/root,root-mirror/root,agarciamontoro/root,georgtroska/root,CristinaCristescu/root,thomaskeck/root,omazapa/root-old,gganis/root,arch1tect0r/root,perovic/root,zzxuanyuan/root,bbockelm/root,gganis/root,0x0all/ROOT,jrtomps/root,karies/root,mhuwiler/rootauto,perovic/root,karies/root,BerserkerTroll/root,perovic/root,davidlt/root,jrtomps/root,abhinavmoudgil95/root,dfunke/root,thomaskeck/root,agarciamontoro/root,Y--/root,gganis/root,bbockelm/root,Duraznos/root,root-mirror/root,davidlt/root,zzxuanyuan/root,thomaskeck/root,sbinet/cxx-root,esakellari/root,zzxuanyuan/root,omazapa/root-old,agarciamontoro/root,sawenzel/root,mhuwiler/rootauto,simonpf/root,gbitzes/root,georgtroska/root,esakellari/my_root_for_test,BerserkerTroll/root,arch1tect0r/root,sirinath/root,agarciamontoro/root,agarciamontoro/root,Y--/root,zzxuanyuan/root,esakellari/my_root_for_test,satyarth934/root,gganis/root,gbitzes/root,esakellari/root,sawenzel/root,CristinaCristescu/root,krafczyk/root,smarinac/root,pspe/root,BerserkerTroll/root,sirinath/root,krafczyk/root,nilqed/root,agarciamontoro/root,cxx-hep/root-cern,satyarth934/root,vukasinmilosevic/root,zzxuanyuan/root-compressor-dummy,cxx-hep/root-cern,sawenzel/root,sbinet/cxx-root,buuck/root,sirinath/root,omazapa/root-old,omazapa/root-old,karies/root,zzxuanyuan/root-compressor-dummy,mattkretz/root,gganis/root,bbockelm/root,simonpf/root,jrtomps/root,olifre/root,zzxuanyuan/root,cxx-hep/root-cern,beniz/root,gganis/root,pspe/root,Duraznos/root,krafczyk/root,dfunke/root,sirinath/root,sbinet/cxx-root,lgiommi/root,agarciamontoro/root,sirinath/root,krafczyk/root,Y--/root,esakellari/my_root_for_test,simonpf/root,georgtroska/root,nilqed/root,satyarth934/root,satyarth934/root,omazapa/root-old,lgiommi/root,BerserkerTroll/root,simonpf/root,mattkretz/root,zzxuanyuan/root-compressor-dummy,dfunke/root,lgiommi/root,arch1tect0r/root,georgtroska/root,zzxuanyuan/root-compressor-dummy,davidlt/root,sirinath/root,gganis/root,simonpf/root,vukasinmilosevic/root,sbinet/cxx-root,perovic/root,karies/root,nilqed/root,abhinavmoudgil95/root,cxx-hep/root-cern,georgtroska/root,root-mirror/root,esakellari/my_root_for_test,mkret2/root,pspe/root,esakellari/my_root_for_test,karies/root,dfunke/root,gbitzes/root,gganis/root,omazapa/root-old,buuck/root,evgeny-boger/root,BerserkerTroll/root,zzxuanyuan/root-compressor-dummy,olifre/root,georgtroska/root,perovic/root,sawenzel/root,gbitzes/root,olifre/root,lgiommi/root,veprbl/root,davidlt/root,karies/root,bbockelm/root,beniz/root,georgtroska/root,olifre/root,pspe/root,evgeny-boger/root,lgiommi/root,0x0all/ROOT,sirinath/root,georgtroska/root,omazapa/root,gbitzes/root,simonpf/root,sbinet/cxx-root,root-mirror/root,esakellari/root,buuck/root,bbockelm/root,0x0all/ROOT,omazapa/root,Duraznos/root,simonpf/root,smarinac/root,gbitzes/root,mhuwiler/rootauto,mkret2/root,nilqed/root,zzxuanyuan/root