ENH: Configure ExternalData local object stores for ITK
ENH: Configure ExternalData local object stores for ITK

Expose ExternalData option ExternalData_OBJECT_STORES as a cache entry
defaulting to the empty string.  If not set, use a directory in the
build tree as the main store.  Either way, add the .ExternalData
directory at the top of the source tree as a local object store.  Remove
the latter from the URL templates list so that the objects can be used
without copying to another local store.

Change-Id: I0b454872ad4855e28bd03351c41b6cd02f107204

jmerkow/ITK,jcfr/ITK,hjmjohnson/ITK,BRAINSia/ITK,blowekamp/ITK,zachary-williamson/ITK,stnava/ITK,hjmjohnson/ITK,fuentesdt/InsightToolkit-dev,spinicist/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,hendradarwin/ITK,InsightSoftwareConsortium/ITK,blowekamp/ITK,PlutoniumHeart/ITK,fuentesdt/InsightToolkit-dev,heimdali/ITK,blowekamp/ITK,stnava/ITK,jcfr/ITK,BlueBrain/ITK,zachary-williamson/ITK,LucHermitte/ITK,jcfr/ITK,BlueBrain/ITK,richardbeare/ITK,biotrump/ITK,paulnovo/ITK,BlueBrain/ITK,blowekamp/ITK,biotrump/ITK,BlueBrain/ITK,PlutoniumHeart/ITK,GEHC-Surgery/ITK,fedral/ITK,eile/ITK,hjmjohnson/ITK,spinicist/ITK,LucHermitte/ITK,zachary-williamson/ITK,rhgong/itk-with-dom,jmerkow/ITK,fuentesdt/InsightToolkit-dev,PlutoniumHeart/ITK,stnava/ITK,biotrump/ITK,hendradarwin/ITK,fedral/ITK,heimdali/ITK,rhgong/itk-with-dom,fedral/ITK,eile/ITK,fuentesdt/InsightToolkit-dev,stnava/ITK,msmolens/ITK,ajjl/ITK,GEHC-Surgery/ITK,LucasGandel/ITK,LucasGandel/ITK,eile/ITK,atsnyder/ITK,eile/ITK,hinerm/ITK,LucasGandel/ITK,rhgong/itk-with-dom,fuentesdt/InsightToolkit-dev,malaterre/ITK,BlueBrain/ITK,hendradarwin/ITK,Kitware/ITK,thewtex/ITK,spinicist/ITK,atsnyder/ITK,jcfr/ITK,PlutoniumHeart/ITK,atsnyder/ITK,rhgong/itk-with-dom,hjmjohnson/ITK,LucasGandel/ITK,BRAINSia/ITK,LucHermitte/ITK,rhgong/itk-with-dom,fedral/ITK,spinicist/ITK,GEHC-Surgery/ITK,jcfr/ITK,LucHermitte/ITK,zachary-williamson/ITK,biotrump/ITK,jcfr/ITK,heimdali/ITK,malaterre/ITK,vfonov/ITK,paulnovo/ITK,msmolens/ITK,PlutoniumHeart/ITK,rhgong/itk-with-dom,msmolens/ITK,atsnyder/ITK,InsightSoftwareConsortium/ITK,zachary-williamson/ITK,heimdali/ITK,LucasGandel/ITK,CapeDrew/DCMTK-ITK,eile/ITK,hinerm/ITK,msmolens/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK,BRAINSia/ITK,paulnovo/ITK,ajjl/ITK,fuentesdt/InsightToolkit-dev,richardbeare/ITK,BlueBrain/ITK,ajjl/ITK,blowekamp/ITK,CapeDrew/DCMTK-ITK,jmerkow/ITK,msmolens/ITK,Kitware/ITK,eile/ITK,LucHermitte/ITK,paulnovo/ITK,LucasGandel/ITK,heimdali/ITK,LucHermitte/ITK,zachary-williamson/ITK,msmolens/ITK,stnava/ITK,fbudin69500/ITK,hjmjohnson/ITK,jmerkow/ITK,blowekamp/ITK,atsnyder/ITK,blowekamp/ITK,jmerkow/ITK,jmerkow/ITK,spinicist/ITK,richardbeare/ITK,paulnovo/ITK,thewtex/ITK,hendradarwin/ITK,vfonov/ITK,malaterre/ITK,malaterre/ITK,thewtex/ITK,biotrump/ITK,fedral/ITK,BRAINSia/ITK,zachary-williamson/ITK,eile/ITK,fuentesdt/InsightToolkit-dev,hinerm/ITK,BlueBrain/ITK,CapeDrew/DCMTK-ITK,msmolens/ITK,spinicist/ITK,vfonov/ITK,ajjl/ITK,fbudin69500/ITK,jcfr/ITK,vfonov/ITK,Kitware/ITK,Kitware/ITK,hinerm/ITK,thewtex/ITK,spinicist/ITK,atsnyder/ITK,biotrump/ITK,malaterre/ITK,hinerm/ITK,vfonov/ITK,biotrump/ITK,atsnyder/ITK,malaterre/ITK,CapeDrew/DCMTK-ITK,rhgong/itk-with-dom,hendradarwin/ITK,paulnovo/ITK,fbudin69500/ITK,GEHC-Surgery/ITK,fbudin69500/ITK,stnava/ITK,richardbeare/ITK,Kitware/ITK,heimdali/ITK,fedral/ITK,fedral/ITK,vfonov/ITK,Kitware/ITK,stnava/ITK,richardbeare/ITK,BlueBrain/ITK,fedral/ITK,GEHC-Surgery/ITK,paulnovo/ITK,eile/ITK,hinerm/ITK,fuentesdt/InsightToolkit-dev,atsnyder/ITK,jmerkow/ITK,CapeDrew/DCMTK-ITK,fbudin69500/ITK,PlutoniumHeart/ITK,PlutoniumHeart/ITK,LucHermitte/ITK,jmerkow/ITK,hjmjohnson/ITK,CapeDrew/DCMTK-ITK,fbudin69500/ITK,GEHC-Surgery/ITK,spinicist/ITK,stnava/ITK,hendradarwin/ITK,hjmjohnson/ITK,malaterre/ITK,heimdali/ITK,heimdali/ITK,LucHermitte/ITK,GEHC-Surgery/ITK,vfonov/ITK,CapeDrew/DCMTK-ITK,ajjl/ITK,fuentesdt/InsightToolkit-dev,hinerm/ITK,Kitware/ITK,ajjl/ITK,zachary-williamson/ITK,thewtex/ITK,InsightSoftwareConsortium/ITK,CapeDrew/DCMTK-ITK,hinerm/ITK,CapeDrew/DCMTK-ITK,hendradarwin/ITK,paulnovo/ITK,BRAINSia/ITK,msmolens/ITK,PlutoniumHeart/ITK,blowekamp/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,jcfr/ITK,LucasGandel/ITK,spinicist/ITK,thewtex/ITK,biotrump/ITK,InsightSoftwareConsortium/ITK,LucasGandel/ITK,atsnyder/ITK,ajjl/ITK,fbudin69500/ITK,vfonov/ITK,stnava/ITK,ajjl/ITK,fbudin69500/ITK,richardbeare/ITK,zachary-williamson/ITK,hendradarwin/ITK,BRAINSia/ITK,eile/ITK,GEHC-Surgery/ITK,malaterre/ITK,hinerm/ITK,malaterre/ITK,rhgong/itk-with-dom