add datatree converters

[OriolAbril]

Description

Adds converters to go back and forth between InferenceData and DataTree.

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--2253.org.readthedocs.build/en/2253/

[codecov]

Codecov Report

Merging #2253 (44b1976) into main (f2d1f11) will decrease coverage by 0.01%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##             main    #2253      +/-   ##
==========================================
- Coverage   87.90%   87.89%   -0.01%     
==========================================
  Files         120      121       +1     
  Lines       12433    12448      +15     
==========================================
+ Hits        10929    10941      +12     
- Misses       1504     1507       +3     

Impacted Files	Coverage Δ
arviz/data/datasets.py	98.48% <ø> (ø)
arviz/data/inference_data.py	82.26% <77.77%> (-0.06%)	⬇️
arviz/data/io_datatree.py	80.00% <80.00%> (ø)
arviz/data/__init__.py	100.00% <100.00%> (ø)