core: Add filtering by index

[coroa]

Fixes #673 .

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

Adding to RELEASE_NOTES.md (remove section after adding to RELEASE_NOTES.md)

Please add a single line in the release notes similar to the following:

- (#XX)[http://link-to-pr.com] Added feature which does something

Description of PR

Tentative implementation for a filter kwarg supporting to select a list of pathways.

[coroa]

Needs documentation and a test. Extend when you have the time. I'll try to do something about it tmw.

[coroa]

Failing tests are iiasa and unfcc connectors. Pbb unrelated to this PR.

[codecov]

Codecov Report

Merging #674 (56a9bc0) into main (52b4d03) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #674   +/-   ##
=====================================
  Coverage   94.7%   94.7%           
=====================================
  Files         59      59           
  Lines       5768    5800   +32     
=====================================
+ Hits        5463    5495   +32     
  Misses       305     305           

Impacted Files	Coverage Δ
pyam/core.py	94.6% <100.0%> (+<0.1%)	⬆️
tests/test_filter.py	100.0% <100.0%> (ø)

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[coroa]

Failing tests are iiasa and unfcc connectors. Pbb unrelated to this PR.

Thanks for fixing, @danielhuppmann . Incorporated.

[gidden]

I've added minimal docstrings and to the release notes. @coroa this looks good to go from my side - good for you?

[gidden]

And any final thoughts from @danielhuppmann also welcome. Unless there's more to add, will merge when tests pass.

[danielhuppmann]

One minor suggestion to add the pandas.MultiIndex explicitly to the docs, otherwise - awesome!

[gidden]

Thank you @coroa !

[coroa]

Good to go from my side :)