Add option to filter by measurand

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• 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 IAMconsortium/nomenclature#360

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@          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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[phackstock]

One small comment, after that good to merge.