Add draft of 1.9 api changes for the flow package.
Add draft of 1.9 api changes for the flow package.

Based on the previous drafts posted at #1162 .

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