Test genno vs. legacy reporting

[khaeru]

This PR adds:

1. A CSV file with the current output of the legacy reporting, run on snapshot-1.
2. A new test, .test_report.test_compare(), and associated utility function that:
   • Runs the genno-based reporting on the stored solution data from snapshot-1.
   • Compares the output to the stored output from (1).

This functions as a test of known differences between the output of the legacy and genno-based reporting for this particular scenario. The added variables NOT_IMPLEMENTED_MEASURE and NOT_IMPLEMENTED_IAMC can be modified and reduced as the genno-based reporting is expanded towards feature parity with the legacy reporting.

Also:

• Refine handling of "iamc:" sections in the reporting config YAML file.
• Extend the legacy reporting config file default_units.yaml to suppress some of the many warnings about missing "conversion factors".
• Add .report.Config.modules and .iter_callbacks().
• Add at key "all::iamc" a task that concatenates IAMC-structured inputs; this can be extended by various modules (e.g. as is done in .compat).

How to review

• Read the changes in “What's new” and additions to the docs.
• Skim the added test.
• Note that 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 91.46341% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.7%. Comparing base (2bc42a3) to head (f943878).
⚠️ Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
message_ix_models/tools/iamc.py	82.9%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #178     +/-   ##
=======================================
- Coverage   75.3%   74.7%   -0.7%     
=======================================
  Files        274     274             
  Lines      22492   22545     +53     
=======================================
- Hits       16958   16856    -102     
- Misses      5534    5689    +155     

Files with missing lines	Coverage Δ
message_ix_models/cli.py	93.4% <ø> (ø)
message_ix_models/report/__init__.py	92.3% <100.0%> (+0.4%)	⬆️
message_ix_models/report/compat.py	99.1% <100.0%> (-0.1%)	⬇️
message_ix_models/report/config.py	100.0% <100.0%> (ø)
message_ix_models/report/sim.py	98.5% <100.0%> (ø)
message_ix_models/tests/report/test_legacy.py	50.0% <ø> (-50.0%)	⬇️
message_ix_models/tests/test_report.py	100.0% <100.0%> (ø)
message_ix_models/tools/add_dac/__init__.py	67.4% <100.0%> (ø)
message_ix_models/util/_message_ix.py	100.0% <100.0%> (ø)
message_ix_models/tools/iamc.py	91.5% <82.9%> (-5.4%)	⬇️

... and 6 files with indirect coverage changes

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[khaeru]

Hi @macflo8 @Wegatriespython @macflo8 @junukitashepard —as mentioned yesterday, I've brought this PR up to date and got it ready to merge. I've requested a review mainly "FYI".

Later, as the scope of IAMC output from the genno reporting grows, I will show how to adjust the NOT_IMPLEMENTED_{MEASURE,IAMC} variables (essentially, remove or edit entries) such that the test still passes.

@yiyi1991 FYI those variables function as the checklist you mentioned in Slack.

Only one ✅ is needed to merge.

[macflo8]

Thanks for adding this. Looks ready be merged!