added unit test for duplication_divergence_graph
added unit test for duplication_divergence_graph

RMKD/networkx,debsankha/networkx,ghdk/networkx,nathania/networkx,jni/networkx,farhaanbukhsh/networkx,RMKD/networkx,dmoliveira/networkx,sharifulgeo/networkx,SanketDG/networkx,ghdk/networkx,goulu/networkx,kernc/networkx,harlowja/networkx,blublud/networkx,aureooms/networkx,tmilicic/networkx,kernc/networkx,aureooms/networkx,dhimmel/networkx,dmoliveira/networkx,jakevdp/networkx,ghdk/networkx,harlowja/networkx,debsankha/networkx,NvanAdrichem/networkx,harlowja/networkx,blublud/networkx,aureooms/networkx,JamesClough/networkx,sharifulgeo/networkx,dmoliveira/networkx,jni/networkx,andnovar/networkx,ionanrozenfeld/networkx,dhimmel/networkx,jfinkels/networkx,wasade/networkx,chrisnatali/networkx,Sixshaman/networkx,debsankha/networkx,beni55/networkx,nathania/networkx,farhaanbukhsh/networkx,dhimmel/networkx,ionanrozenfeld/networkx,bzero/networkx,ionanrozenfeld/networkx,michaelpacer/networkx,blublud/networkx,jcurbelo/networkx,bzero/networkx,nathania/networkx,farhaanbukhsh/networkx,OrkoHunter/networkx,ltiao/networkx,RMKD/networkx,jakevdp/networkx,yashu-seth/networkx,kernc/networkx,chrisnatali/networkx,cmtm/networkx,jni/networkx,chrisnatali/networkx,sharifulgeo/networkx,bzero/networkx,jakevdp/networkx