Add metadata lookup in parent DataTree attrs to core rules

[clarasb]

This PR adds metadata lookup in parent DataTree attrs to core rules.

Some metadata rules check “global” attrs, but these attrs can live in a parent/root group while the data lives in a child group. The changes in this PR let the rule content-desc and conventions, look up attrs from parent DataTree groups without copying them into child datasets. Local attrs still take precedence, so the physical group structure stays unchanged.

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 99.49%. Comparing base (57fb020) to head (67492c4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #73   +/-   ##
=======================================
  Coverage   99.48%   99.49%           
=======================================
  Files          74       75    +1     
  Lines        2935     2967   +32     
  Branches      426      432    +6     
=======================================
+ Hits         2920     2952   +32     
  Partials       15       15           

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