added the Katz centrality with tests
Update the code documentation.
Update the code documentation.
Update the code documentation.
Many small updates to Katz centrality.

Addresses #767

- Add normalization option (what is the standard normalization?).
- Beta parameter default=1.0 and converts to dict or array, need to add option to specify full input of beta for each node
- Doc fixes in many places
More housekeeping.

- Use decorators for testing input graph types
- Return empty dict for empty graph
- Simplify numpy imports
Use numpy.linalg.solve instead of inverting matrix.
Whitespace cleanup.
Doc fixes
add the following test test_beta_as_scalar, test_beta_as_dict,
test_beta_as_vector, test_multiple_alpha
update the biblio
Merge pull request #2 from vgauthier/767-katz

add the following test test_beta_as_scalar, test_beta_as_dict,
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.
Merge pull request #819 from hagberg/767-katz

Katz Centrality (update from #767)

Fixes #767