Implement an `apply()` function

[pjuergens]

Please confirm that this PR has done the following:

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

Description of PR

Implemented an apply() function, closes #528

[codecov[bot]]

Codecov Report

Merging #533 (66a0106) into main (63bbea8) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #533   +/-   ##
=====================================
  Coverage   93.4%   93.4%           
=====================================
  Files         45      47    +2     
  Lines       5074    5123   +49     
=====================================
+ Hits        4740    4789   +49     
  Misses       334     334           

Impacted Files	Coverage Δ
pyam/_ops.py	100.0% <100.0%> (ø)
pyam/core.py	92.6% <100.0%> (+<0.1%)	:arrow_up:
tests/test_ops.py	100.0% <100.0%> (ø)
tests/test_feature_debiasing.py	100.0% <0.0%> (ø)
pyam/_debiasing.py	100.0% <0.0%> (ø)

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[pjuergens]

It should be similar now to pandas apply with the possibility of both args (in my implementation before components) and **kwargs (in pandas-style however **kwds)

[danielhuppmann]

FYI, I added the description of this PR to directly close the related issue when this PR gets merged

[pjuergens]

Should be ready to go now :) I had to put the _get_values function inside _ops.py, because I could load get_index_levels from withing utils.py.

[danielhuppmann]

Should be ready to go now :) I had to put the _get_values function inside _ops.py, because I could load get_index_levels from withing utils.py.

Yeah, one has to be careful about circular dependencies... But it's fine to have that close to the actual use case.