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
Merge remote-tracking branch 'upstream/master'
Bugfix to allow u-v or v-u ordering in edges
Merge pull request #1023 from hagberg/max-matching-order

Max matching order

Fixes #1022