Make categorize and filter work with a custom index

[coroa]

Please confirm that this PR has done the following:

• ~Tests Added~ -> Will be added as part of a test suite for the custom-index feature
• ~Documentation Added~ -> Will be added when the custom-index feature becomes a fully supported feature
• ~Name of contributors Added to AUTHORS.rst~
• ~Description in RELEASE_NOTES.md Added~

Description of PR

Implement the custom-index support in some functions - this is a beta-feature.

[codecov[bot]]

Codecov Report

Merging #582 (cd250cf) into main (151e330) will not change coverage. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #582   +/-   ##
=====================================
  Coverage   93.7%   93.7%           
=====================================
  Files         50      50           
  Lines       5339    5339           
=====================================
  Hits        5004    5004           
  Misses       335     335           

Impacted Files	Coverage Δ
pyam/core.py	94.3% <100.0%> (ø)

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[danielhuppmann]

Looks nice and compact...

[danielhuppmann]

Are you planning any further work on this, @coroa? If not, can you please update the description. (I'm planning to start developing structured tests for custom-index features in the near future, so no need to add specific tests now - keeping in mind that the custom-index is a beta-feature).

[danielhuppmann]

Merging as there was no activity, all tests pass, and it will simplify some follow-up work on my to-do list.