Make `rename()` work with a custom index

[danielhuppmann]

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

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Description of PR

This PR generalizes the rename() function so that it can be used on an IamDataFrame with a custom index (ie., other than ["model", "scenario"], see here.

Two use cases of the custom index:

• retrieving scenario data from the IIASA database API using default=False, which returns all versions of scenarios instead of just the default, and which then uses the custom index ["model", "scenario", "version"], see here
• the openMODEX project (afaik) developed a data format similar to the IAMC template but uses source instead of model. for generality

To-Do

Rather than writing a single test just for this function, I'm thinking about how to redo test_df in the pytest-config so that a custom index can be tested across the entire pyam code base.

[danielhuppmann]

fyi @jh-RLI @christian-rli

[codecov[bot]]

Codecov Report

Merging #561 (e682d84) into main (7d77473) will not change coverage. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #561   +/-   ##
=====================================
  Coverage   93.6%   93.6%           
=====================================
  Files         50      50           
  Lines       5313    5313           
=====================================
  Hits        4978    4978           
  Misses       335     335           

Impacted Files	Coverage Δ
pyam/core.py	94.2% <100.0%> (+<0.1%)	:arrow_up:
tests/conftest.py	100.0% <100.0%> (ø)

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[danielhuppmann]

Working on this further, I realized that restructuring the test suite and simultaneously fixing all issues where the custom-index-feature breaks is too much. Hence, I'm changing tactics - trusting that the existing tests cover all officially supported features, extending features to support custom-index IamDataFrame instances, then extending the test_df to also cover a case with a custom-index