Adjust .ssp.transport to avoid negative emissions totals

[khaeru]

Closes #387.

The initial commits implement the first method described at that issue, from the description of #387. These were made before the method was changed again; the later commits [will] implement the second method.

How to review

• Read the diff.
• Note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update doc/whatsnew.

[codecov]

Codecov Report

❌ Patch coverage is 89.06250% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.2%. Comparing base (945afcb) to head (0335c1f).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
message_ix_models/project/ssp/transport.py	85.7%	6 Missing ⚠️
...sage_ix_models/tests/project/ssp/test_transport.py	95.2%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #389     +/-   ##
=======================================
- Coverage   76.9%   76.2%   -0.8%     
=======================================
  Files        246     246             
  Lines      19866   19897     +31     
=======================================
- Hits       15279   15162    -117     
- Misses      4587    4735    +148     

Files with missing lines	Coverage Δ
...age_ix_models/tests/model/transport/test_report.py	94.1% <100.0%> (+<0.1%)	⬆️
...sage_ix_models/tests/project/ssp/test_transport.py	99.1% <95.2%> (-0.9%)	⬇️
message_ix_models/project/ssp/transport.py	97.0% <85.7%> (+0.7%)	⬆️

... and 7 files with indirect coverage changes

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