make dm-tree optional

[OriolAbril]

Description

dm-tree dependency is only used to support pytree objects like nested dicts which
is not very common and is also very localized in the codebase, being only used via
either dict_to_dataset or from_dict (both also having the dict->pytree alternative naming).

I would like to keep dependencies for base functionality to something more basic so
we can for example use arviz via pyodide. Also in preparation to arviz-xyz refactoring
where anything not required throughout the library is an optional dependency.

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--2379.org.readthedocs.build/en/2379/

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Project coverage is 87.00%. Comparing base (5a928fb) to head (a42010c).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
arviz/data/base.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2379   +/-   ##
=======================================
  Coverage   86.99%   87.00%           
=======================================
  Files         124      124           
  Lines       12862    12877   +15     
=======================================
+ Hits        11189    11203   +14     
- Misses       1673     1674    +1     

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[ColCarroll]

Looks good to me!

I picked dm-tree as less weird of a dependency than jax, but the latter is probably way more common (especially if the user has a pytree or nested dict). I might open an issue (or just make the PR) to allow tree to be either dm-tree or jax.tree.

[OriolAbril]

to allow tree to be either dm-tree or jax.tree.

That would be nice