Enable reading non-default scenario versions from IIASA API

[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 uses the "custom-index" feature (currently in beta support, see #481) to read data of non-default scenario versions from the IIASA RestAPI and return an IamDataFrame with a non-standard index.

[danielhuppmann]

@fbenke-pik, you may like this new feature... Try pyam.read_iiasa(<instance>, default=False) to get not just the latest version of a model-scenario but all of versions uploaded to the IIASA database...

[codecov]

Codecov Report

Merging #486 (698b7af) into master (3d31fdd) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #486      +/-   ##
==========================================
+ Coverage   92.96%   92.99%   +0.03%     
==========================================
  Files          41       41              
  Lines        4746     4756      +10     
==========================================
+ Hits         4412     4423      +11     
+ Misses        334      333       -1     

Impacted Files	Coverage Δ
pyam/iiasa.py	89.18% <100.00%> (+0.30%)	⬆️
tests/test_iiasa.py	92.80% <100.00%> (+0.67%)	⬆️

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[danielhuppmann]

Merging as there were no critical comments or objections...