optional normalization for psislw

[mdmould]

Description

Adds option to not normalize log weights from Pareto smoothed importance sampling. This is useful when the scale of the weights are to be preserved (e.g., importance sampling against an unnormalized distribution).

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

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📚 Documentation preview 📚: https://arviz--2455.org.readthedocs.build/en/2455/

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.79%. Comparing base (1b0b9cb) to head (d2ce305).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
arviz/stats/stats.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2455      +/-   ##
==========================================
- Coverage   86.79%   86.79%   -0.01%     
==========================================
  Files         124      124              
  Lines       12995    12998       +3     
==========================================
+ Hits        11279    11281       +2     
- Misses       1716     1717       +1     

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[OriolAbril]

Thanks!