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 variable IGNORE can be extended and modified 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

TBA

PR checklist

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

[codecov]

Codecov Report

Attention: Patch coverage is 30.30303% with 46 lines in your changes missing coverage. Please review.

Project coverage is 55.5%. Comparing base (4fd6446) to head (1034331).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
message_ix_models/tests/test_report.py	21.8%	43 Missing ⚠️
message_ix_models/report/config.py	60.0%	2 Missing ⚠️
message_ix_models/report/compat.py	0.0%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (4fd6446) and HEAD (1034331). Click for more details.

HEAD has 12 uploads less than BASE

Flag	BASE (4fd6446)	HEAD (1034331)
	15	3

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #178      +/-   ##
========================================
- Coverage   77.4%   55.5%   -21.9%     
========================================
  Files        217     217              
  Lines      16851   16907      +56     
========================================
- Hits       13048    9395    -3653     
- Misses      3803    7512    +3709     

Files with missing lines	Coverage Δ
message_ix_models/cli.py	93.4% <ø> (ø)
message_ix_models/report/__init__.py	87.3% <100.0%> (-8.4%)	⬇️
message_ix_models/tests/report/test_legacy.py	55.0% <ø> (ø)
message_ix_models/report/compat.py	35.1% <0.0%> (-64.2%)	⬇️
message_ix_models/report/config.py	94.1% <60.0%> (-5.9%)	⬇️
message_ix_models/tests/test_report.py	68.2% <21.8%> (-31.8%)	⬇️

... and 65 files with indirect coverage changes

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