Add option to filter by `measurand`

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Tests Added
• [x] Documentation Added
• ~Name of contributors Added to AUTHORS.rst~
• [x] Description in RELEASE_NOTES.md Added

Description of PR

This PR adds a feature to support filtering by a "measurand", i.e., a tuple (or iterable of tuples) of variable and unit

See the related discussion at https://github.com/IAMconsortium/nomenclature/pull/360

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.0%. Comparing base (ddbb88e) to head (fd5338e). Report is 25 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #868 +/- ## ===================================== Coverage 95.0% 95.0% ===================================== Files 64 64 Lines 6134 6191 +57 ===================================== + Hits 5828 5884 +56 - Misses 306 307 +1 ```

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