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 87.14286% with 9 lines in your changes missing coverage. Please review.

Project coverage is 58.9%. Comparing base (5270640) to head (7bc4db1).
Report is 2 commits behind head on main.

Files	Patch %	Lines
message_ix_models/tests/test_report.py	83.9%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #178     +/-   ##
=======================================
+ Coverage   58.8%   58.9%   +0.1%     
=======================================
  Files        194     194             
  Lines      15159   15224     +65     
=======================================
+ Hits        8922    8978     +56     
- Misses      6237    6246      +9     

Files	Coverage Δ
message_ix_models/cli.py	93.4% <ø> (ø)
message_ix_models/report/__init__.py	97.4% <100.0%> (+<0.1%)	⬆️
message_ix_models/report/compat.py	99.2% <100.0%> (+<0.1%)	⬆️
message_ix_models/report/config.py	100.0% <100.0%> (ø)
message_ix_models/tests/report/test_legacy.py	55.0% <ø> (ø)
message_ix_models/tests/test_report.py	94.4% <83.9%> (-5.6%)	⬇️