Update Katz centrality code+docs

Rearrange docs to put definition at top.  Fix formatting.

Only allow beta to be a dict or scalar.  Do checking to make sure it is specified properly to avoid confusing errors later one if e.g. beta doesn't contain all of the nodes n the graph.

Split numpy tests into separate class so non-numpy version gets tested even without numpy installation.  Add more tests including comparison to eigenvector centrality.
Fix example.