[FIX] Add MultiIndex support

[m-marqx]

One thing I recently noticed is that when a dataframe has MultiIndex columns, the method corr_interactive_plot always results in a dataframe with missing labels:

This error occurs with this method because Plotly struggles with dataframes that have MultiIndex columns. This fix should resolve the missing labels problem by merging the MultiIndex into a list, following the Seaborn format seen in corr_plot, to maintain the graph styles.

With this fix:

I don't know if this change creates a bug with dataframes that have a MultiIndex index, resulting in an error or a bugged plot, but I can't imagine a real situation where a dataframe would have MultiIndex indexes. However, MultiIndex columns could be possible in very specific situations.

[sonarqubecloud]

Quality Gate passed

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[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (6328744) 51.18% compared to head (de59319) 50.58%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/klib/describe.py	14.28%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
- Coverage   51.18%   50.58%   -0.60%     
==========================================
  Files           3        3              
  Lines         508      514       +6     
  Branches       68       72       +4     
==========================================
  Hits          260      260              
- Misses        242      248       +6     
  Partials        6        6              

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