Use "left eigenvector" (in-edges) in centrality

Change both Katz centrality and eigenvector centrality for directed graphs to use "left eigenvector" centrality (in-edges).
Use G.reverse() to get out-edge centrality.
Merge pull request #1020 from hagberg/left-eigenvector

Use "left eigenvector" (in-edges) in centrality

Fixes #1016