Miscellaneous enhancements for 2023-W36

[khaeru]

• New module .project.ssp to:
  • Generate SDMX codelists for the 2017/original SSPs and the 2024 update.
  • Provide these as enums to other code.
• New module .tools.exo_data to retrieve exogenous data for, for instance, population and GDP.
• New function .model.emissions.get_emission_factors() and associated data file to provide data from this table in message_doc.
• New functions in .util.sdmx:
  • read(), write() to retrieve/store package data in SDMX-ML.
  • make_enum() to make pure-Python Enum (or subclass) data structures based on SDMX code lists.
• same_node(): also fill "node_shares", "node_loc", and "node", as appropriate.
• Use pre-commit to invoke mypy, black, and ruff; see Use pre-commit to invoke ruff, mypy message_ix#742.

Housekeeping:

• Revert workaround for Adjust CI for missing codecov #96 in order to reactivate tests for upstream versions 3.4, 3.5, and 3.6.
• Numerous fixes to Sphinx "nitpicks": warnings about missing references. These are reduced by more than 50%, but many still remain. The -n flag is added to the test docs build in the CI workflow so we can see an up-to-date list of these.

How to review

Read the diff and note that the CI checks all pass.

PR checklist

• Merge required changes upstream
  • Handle units like "kg / MJ" in convert_gwp() IAMconsortium/units#45 —and release
  • Miscellaneous enhancements for 2023-W36 khaeru/genno#103
  • Release v1.19.0 khaeru/genno#104
• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update doc/whatsnew.

[codecov]

Codecov Report

Merging #122 (4074c83) into main (7eb2a16) will increase coverage by 2.1%.
The diff coverage is 99.1%.

@@           Coverage Diff           @@
##            main    #122     +/-   ##
=======================================
+ Coverage   67.1%   69.3%   +2.1%     
=======================================
  Files         70      75      +5     
  Lines       4640    4975    +335     
=======================================
+ Hits        3116    3448    +332     
- Misses      1524    1527      +3     

Files Changed	Coverage Δ
message_ix_models/cli.py	100.0% <ø> (ø)
message_ix_models/model/bare.py	100.0% <ø> (ø)
message_ix_models/model/disutility.py	100.0% <ø> (ø)
message_ix_models/model/macro.py	100.0% <ø> (ø)
message_ix_models/model/structure.py	100.0% <ø> (ø)
message_ix_models/report/util.py	51.8% <ø> (ø)
message_ix_models/testing.py	100.0% <ø> (ø)
message_ix_models/tests/model/test_bare.py	100.0% <ø> (ø)
message_ix_models/tests/model/test_disutility.py	100.0% <ø> (ø)
message_ix_models/tests/test_cli.py	100.0% <ø> (ø)
... and 27 more