Fix propagation of global attributes to child datasets and variables

[clarasb]

This PR fixes the propagation of global DataTree attributes to child Datasets and Variables.

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 99.46%. Comparing base (cbaf81a) to head (c84f97f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #72   +/-   ##
=======================================
  Coverage   99.46%   99.46%           
=======================================
  Files          67       67           
  Lines        2810     2816    +6     
  Branches      413      413           
=======================================
+ Hits         2795     2801    +6     
  Partials       15       15           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.