Don't recompute Wiener index unless necessary.
Don't recompute Wiener index unless necessary.

This commit changes the behavior of the `closeness_vitality()` function
so that it only computes the Wiener index of the original graph once
instead of recomputing the same value for each node in the graph.

JamesClough/networkx,cmtm/networkx,jfinkels/networkx,Sixshaman/networkx,goulu/networkx,NvanAdrichem/networkx,andnovar/networkx