Df load meta

[gidden]

Please confirm that this PR has done the following:

• [ ] Tests Added - existing tests cover use case
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Adding to RELEASE_NOTES.md (remove section after adding to RELEASE_NOTES.md)

Please add a single line in the release notes similar to the following:

- (#XX)[http://link-to-pr.com] Added feature which does something

Description of PR

This PR implements support for setting metadata by dataframes, and implements all metadata setting solely via set_meta()

[codecov]

Codecov Report

Merging #679 (776494b) into main (74979e7) will decrease coverage by 0.0%.
The diff coverage is 85.7%.

@@           Coverage Diff           @@
##            main    #679     +/-   ##
=======================================
- Coverage   94.7%   94.7%   -0.1%     
=======================================
  Files         59      59             
  Lines       5800    5812     +12     
=======================================
+ Hits        5495    5506     +11     
- Misses       305     306      +1     

Impacted Files	Coverage Δ
pyam/core.py	94.6% <85.7%> (-0.1%)	⬇️
pyam/iiasa.py	85.8% <0.0%> (ø)
pyam/aggregation.py	99.1% <0.0%> (+<0.1%)	⬆️
tests/test_feature_aggregate.py	98.9% <0.0%> (+<0.1%)	⬆️

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[danielhuppmann]

Thanks @gidden! One side effect of your clean-up is that the warnings are no longer shown if the (timeseries) data sheet and the meta of a file are inconsistent. Given that we are starting to use the meta-indicators-feature that more in the scenario-submission workflows, I'd be very much in favor of keeping the warnings.

[gidden]

Changes to load_meta() reverted!

[danielhuppmann]

Thanks @gidden, lgtm!

[gidden]

@danielhuppmann - only style changes, should be g2g

[gidden]

With lgtm above and passing tests, merging