Fix PRICE_EMISSION definition

[glatterf42]

In preparation of the 3.10 release, @khaeru and I noticed that the original PR to fix PRICE_EMISSION (address #723) has become a little cluttered with discussions of how to create a proper test for the calculation update. Meanwhile, @volker-krey suggested that we should just bring the fix to main and open a follow-up issue to create the test later. Thus, this PR adds all changes from #726 that seem to be necessary to fix the calculation and make the tests pass as they are.

Caution

The error with the current calculation of PRICE_EMISSION went undetected so long because our tests were not stringent enough. Hence our initial insistence that #726 contain the exact kind of test to prevent the issue from coming up again. Such a test is missing from this PR on purpose, but that makes it incomplete in that sense. We need to maintain the sense of urgency that this test should be added as quickly as possible.
The only reason we are considering merging this without the test is because both @OFR-IIASA and @yiyi1991 manually confirmed that this fix works on sufficiently complex scenarios.

How to review

• Read the diff and note that the CI checks all pass.
• Ensure that changes/additions are self-documenting, i.e. that another
  developer (someone like the reviewer) will be able to understand what the code
  does in the future.
• If you know of further places in our docs where the PRICE_EMISSION calculation is explained and needs updating, please share that.

PR checklist

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

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 10 lines in your changes missing coverage. Please review.

Project coverage is 94.6%. Comparing base (e0f2e02) to head (8de6e41).

Files with missing lines	Patch %	Lines
message_ix/tests/test_feature_price_emission.py	64.2%	10 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #912      +/-   ##
========================================
+ Coverage   24.0%   94.6%   +70.5%     
========================================
  Files         48      48              
  Lines       4394    4456      +62     
========================================
+ Hits        1057    4216    +3159     
+ Misses      3337     240    -3097     

Files with missing lines	Coverage Δ
message_ix/models.py	99.0% <ø> (+14.5%)	⬆️
message_ix/tests/test_report.py	100.0% <100.0%> (+79.6%)	⬆️
message_ix/tests/test_tutorials.py	97.6% <ø> (ø)
message_ix/tests/test_feature_price_emission.py	72.6% <64.2%> (+61.9%)	⬆️

... and 42 files with indirect coverage changes