Remove unused meta levels during initialization

[danielhuppmann]

Please confirm that this PR has done the following:

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

Description of PR

This PR fixes a bug where the IamDataFrame attributes (model, scenario, ...) where not correctly downselected when initializing an IamDataFrame with a meta dataframe that had additional model-scenario combinations.

closes #694

[codecov[bot]]

Codecov Report

Merging #695 (2015bfc) into main (bd3aea4) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #695   +/-   ##
=====================================
  Coverage   94.8%   94.8%           
=====================================
  Files         58      58           
  Lines       5819    5823    +4     
=====================================
+ Hits        5521    5525    +4     
  Misses       298     298           

Impacted Files	Coverage Δ
pyam/core.py	94.8% <100.0%> (+<0.1%)	:arrow_up:
tests/test_core.py	100.0% <100.0%> (ø)
tests/test_iiasa.py	97.2% <100.0%> (+<0.1%)	:arrow_up:

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