Updated the return type of Katz centrality

[nwlandry]

This PR accomplishes the following:

• katz_centrality now returns a dictionary to be consistent with all other centrality measures.
• The Katz centrality is now 1-normalized consistent with other functions
• katz_centrality is now a nodestat as well.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 92.12%. Comparing base (369a10a) to head (eb3258b).

Files	Patch %	Lines
xgi/stats/nodestats.py	33.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #530 +/- ## ========================================== - Coverage 92.19% 92.12% -0.07% ========================================== Files 60 60 Lines 4392 4393 +1 ========================================== - Hits 4049 4047 -2 - Misses 343 346 +3 ```

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[nwlandry]

Looks good! We should mention that the values that are returned are normalised, can you add that?

I have mentioned that centralities are 1-normalized in the Returns field.

[nwlandry]

Thanks for the review!!