add class method GEXFWriter.alter_graph_mode_timeformat
add class method GEXFWriter.alter_graph_mode_timeformat

extract graph mode and timeformat manipulations to a class method 'GEXFWriter.alter_graph_mode_timeformat', and fix some bugs found in the previous version
cmtm/networkx,debsankha/networkx,jakevdp/networkx,chrisnatali/networkx,harlowja/networkx,debsankha/networkx,jakevdp/networkx,ghdk/networkx,JamesClough/networkx,dhimmel/networkx,sharifulgeo/networkx,harlowja/networkx,dmoliveira/networkx,farhaanbukhsh/networkx,blublud/networkx,kernc/networkx,jakevdp/networkx,farhaanbukhsh/networkx,ionanrozenfeld/networkx,farhaanbukhsh/networkx,harlowja/networkx,jni/networkx,RMKD/networkx,chrisnatali/networkx,ionanrozenfeld/networkx,Sixshaman/networkx,andnovar/networkx,ghdk/networkx,beni55/networkx,jcurbelo/networkx,dmoliveira/networkx,SanketDG/networkx,nathania/networkx,dhimmel/networkx,bzero/networkx,michaelpacer/networkx,aureooms/networkx,bzero/networkx,blublud/networkx,kernc/networkx,dhimmel/networkx,goulu/networkx,RMKD/networkx,nathania/networkx,nathania/networkx,aureooms/networkx,ghdk/networkx,NvanAdrichem/networkx,tmilicic/networkx,ionanrozenfeld/networkx,yashu-seth/networkx,sharifulgeo/networkx,jfinkels/networkx,chrisnatali/networkx,blublud/networkx,bzero/networkx,RMKD/networkx,OrkoHunter/networkx,aureooms/networkx,wasade/networkx,kernc/networkx,debsankha/networkx,jni/networkx,jni/networkx,ltiao/networkx,dmoliveira/networkx,sharifulgeo/networkx