Added support for xarray.DataTree

[forman]

Added support for xarray.DataTree.
Closes #54

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.46%. Comparing base (1210c1e) to head (bd30c33).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #55   +/-   ##
=======================================
  Coverage   99.45%   99.46%           
=======================================
  Files          67       67           
  Lines        2773     2806   +33     
  Branches      408      412    +4     
=======================================
+ Hits         2758     2791   +33     
  Partials       15       15           

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