Fix propagation of global attributes to child datasets and variables

[clarasb]

This PR fixes passing of global datatree attributes to children. Attributes defined on parent datatrees are now inherited by all descendants.

Closes #63.

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 99.48%. Comparing base (353e655) to head (e27d640).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #72   +/-   ##
=======================================
  Coverage   99.48%   99.48%           
=======================================
  Files          74       74           
  Lines        2927     2932    +5     
  Branches      425      425           
=======================================
+ Hits         2912     2917    +5     
  Partials       15       15           

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[forman]

Please fix:

• It is not clear, why the fix is correct. New code is also quite unintuitive.
• Missing unit tests