expand emission scaling parameterization to allow representing GWP*

[volker-krey]

Addition to GAMS code that expands emission scaling parameterization to allow representing GWPstar metric

How to review

• Check the formulation in GAMS equation EMISSION_CONSTRAINT in model_core.gms and comment on adequacy and correctness of equation
• Check the diff for parameter_def.gms and data_load.gms
• Following initial conceptual discussion the other (hidden in this list) steps may need to be performed as part of review

PR checklist

• Continuous integration checks all ✅

• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

Merging #469 (3a3cc43) into main (22de076) will increase coverage by 93.3%.
The diff coverage is n/a.

❗ Current head 3a3cc43 differs from pull request most recent head 53cd050. Consider uploading reports for the commit 53cd050 to get more accurate results

@@           Coverage Diff           @@
##           main    #469      +/-   ##
=======================================
+ Coverage      0   93.3%   +93.3%     
=======================================
  Files         0      37      +37     
  Lines         0    2814    +2814     
=======================================
+ Hits          0    2627    +2627     
- Misses        0     187     +187     

Impacted Files	Coverage Δ
message_ix/reporting/__init__.py	100.0% <0.0%> (ø)
message_ix/__init__.py	89.4% <0.0%> (ø)
message_ix/tests/test_tutorials.py	100.0% <0.0%> (ø)
message_ix/tests/test_core.py	100.0% <0.0%> (ø)
message_ix/tests/test_util.py	100.0% <0.0%> (ø)
message_ix/tests/test_cli.py	100.0% <0.0%> (ø)
message_ix/tools/add_year/__init__.py	66.2% <0.0%> (ø)
message_ix/models.py	100.0% <0.0%> (ø)
message_ix/utils.py	100.0% <0.0%> (ø)
message_ix/core.py	95.2% <0.0%> (ø)
... and 27 more