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

[glatterf42]

We discussed this PR in today's MESSAGE weekly meeting: the changes proposed here where originally intended to be used for a research paper that compares using the standard GWP100 for emission scaling to the GWP* (which e.g. makes methane mitigation more valuable over time). However, this idea fell by the wayside.

Now, no one is actively working on this, though at some point in the future, someone may be interested to pick this back up. Without an active use case (no one actively working on this or using it for something), we don't want to assign anyone to push this over the finish line. Thus, we leave the PR in a draft state.