Split up Bayes Factor stat and plotting functions

[mpmbq2]

Description

This PR addresses issue #2402.

The plot_bf function computes the Bayes Factor statistic, and plots the result. However, it is also useful to just compute the Bayes Factor without making the plot.

This PR splits the Bayes Factor computation off to a function arviz.stats.bayes_factor, which is called by arviz.plots.plot_bf to make the plot.

I see that this PR can't be automatically merged, but it's not immediately clear to me why.

Checklist

• Follows official PR format
• 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--2406.org.readthedocs.build/en/2406/

[aloctavodia]

thanks @mpmbq2!