Fix/concat

[phackstock]

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

closes #664.

The pyam.concat function now carries over the index of the merged meta table. Added a unit test to check under test_feature_append_and_concat.test_concat_non_standard_index. For documentation I added an additional line to the notes section of the pyam.concat function.

One open question from my side would be if there is more documentation than the added docstring required. My hunch would be that it's fine as is since the behavior is not really new but rather more working in line with user expectations.

[codecov[bot]]

Codecov Report

Merging #665 (f95c030) into main (c5e8f9c) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #665   +/-   ##
=====================================
  Coverage   94.5%   94.5%           
=====================================
  Files         59      59           
  Lines       5742    5747    +5     
=====================================
+ Hits        5431    5436    +5     
  Misses       311     311           

Impacted Files	Coverage Δ
pyam/core.py	94.5% <100.0%> (ø)
tests/test_feature_append_concat.py	100.0% <100.0%> (ø)

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[phackstock]

Thanks for the quick review @danielhuppmann. If the tests all run through I'd go ahead with the merge.

[coroa]

Thanks for that! I literally stumbled over it last night.

[phackstock]

Perfect coincidence then, happy to fix it.