Add data providers for SSP data

[khaeru]

• Build on the ExoDataSource class introduced in Miscellaneous enhancements for 2023-W36 #122 to create providers for data from the SSP (original) and SSP Update databases.
  • In order to provide public/non-embargoed data for testing this code, add a CLI command mix-models ssp make-test-data that preserves the structure of the original, but replaces the values with random floats.
• Add a .report.plot submodule with a Plot class and five initial (sets of) plots including:
  • Emissions|CO2
  • Final Energy
  • Primary Energy
• Update and test .report.sim.
  • Add solution data from snapshot 0 for use with add_simulated_solution(). These are excluded from packaging.

How to review

• Read the diff; skip over the .csv.gz files using the GitHub file filter.
• Note the 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

Merging #125 (6a0236d) into main (24c8c58) will increase coverage by 2.6%.
The diff coverage is 90.5%.

@@           Coverage Diff           @@
##            main    #125     +/-   ##
=======================================
+ Coverage   68.1%   70.7%   +2.6%     
=======================================
  Files         75      80      +5     
  Lines       5066    5439    +373     
=======================================
+ Hits        3451    3848    +397     
+ Misses      1615    1591     -24     

Files	Coverage Δ
message_ix_models/project/ssp/__init__.py	95.1% <100.0%> (+0.2%)	⬆️
message_ix_models/project/ssp/data.py	100.0% <100.0%> (ø)
message_ix_models/report/computations.py	52.0% <100.0%> (+2.0%)	⬆️
message_ix_models/report/config.py	100.0% <100.0%> (ø)
message_ix_models/tests/model/test_snapshot.py	100.0% <100.0%> (ø)
message_ix_models/tests/model/test_structure.py	100.0% <100.0%> (ø)
message_ix_models/tests/project/test_ssp.py	100.0% <100.0%> (ø)
...essage_ix_models/tests/report/test_computations.py	100.0% <100.0%> (ø)
message_ix_models/tests/report/test_config.py	100.0% <100.0%> (ø)
message_ix_models/tests/tools/test_exo_data.py	100.0% <100.0%> (ø)
... and 18 more

... and 2 files with indirect coverage changes