Include meta in new IamDataFrames returned by aggregation functions

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Description in RELEASE_NOTES.md Added

Description of PR

This PR ensures that all aggregation functions include the meta table of the original IamDataFrame when returning a new instance (append=False). As part of the implementation, all test dataframes now have one or two meta column, and the merging of meta tables was moved from the append() function to an own function in utils to be reused.

[codecov]

Codecov Report

Merging #416 into master will increase coverage by 0.02%.
The diff coverage is 97.18%.

@@            Coverage Diff             @@
##           master     #416      +/-   ##
==========================================
+ Coverage   93.43%   93.45%   +0.02%     
==========================================
  Files          35       35              
  Lines        4189     4220      +31     
==========================================
+ Hits         3914     3944      +30     
- Misses        275      276       +1     

Impacted Files	Coverage Δ
tests/conftest.py	97.67% <93.75%> (+0.34%)	⬆️
pyam/core.py	92.04% <94.11%> (-0.20%)	⬇️
pyam/utils.py	91.98% <100.00%> (+0.47%)	⬆️
tests/test_feature_aggregate.py	100.00% <100.00%> (ø)
tests/test_feature_append_rename.py	100.00% <100.00%> (ø)
tests/test_iiasa.py	91.59% <100.00%> (-0.08%)	⬇️
tests/test_io.py	100.00% <100.00%> (ø)
tests/test_utils.py	100.00% <100.00%> (ø)

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[znicholls]

All looks fine. The one thing I couldn't see was an explicit test for the behaviour you've changed? Maybe I just missed it though.

[danielhuppmann]

Thanks for the review @znicholls

The one thing I couldn't see was an explicit test for the behaviour you've changed?

The new behavior is tested by adding meta-columns in the test dataframes provided in conftest.py, so the tests check that the new IamDataFrame instances returned by aggregation functions also have these meta indicators (via assert_iamframe_equal()). You could test the test behavior by only extracting the changes to the conftest.py and the test files, but not core.py and utils.py - that should fail.

Anyway, I added an explicit test for merge_meta() to make sure that the precedence of left over right is not accidentally changed in the future.