Makes levels optional in plot_rank_histogram

[mjs2369]

This PR fixes #112 by making the levels argument optional in the plot_rank_histogram function.

It adds logic to skip level binning and plot a single RH if nothing for levels is passed into the fucntion.

It also adds logic to work with identity obs as a possible value for obs_type to be passed into the plot_rank_histogram function.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 43 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.89%. Comparing base (e84b009) to head (cc1d724).

Files with missing lines	Patch %	Lines
src/pydartdiags/matplots/matplots.py	0.00%	43 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
- Coverage   71.78%   69.89%   -1.90%     
==========================================
  Files           4        4              
  Lines         996     1023      +27     
==========================================
  Hits          715      715              
- Misses        281      308      +27     

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[hkershaw-brown]

@mjs2369 can you put this pull request as part of #121.
I cannot run your example without this fix.

[mjs2369]

Closed as moved to PR #121