Add code lists for time periods

[khaeru]

This PR adds two different code lists for time periods, i.e. the year set of MESSAGEix scenarios.

Closes #11.

How to review

Answer the following questions:

• Do the added files match scenarios you are aware of?
• Are you aware of specific scenarios which use different time periods?
• What are they? Please provide complete detail.

PR checklist

• Rebase after Migrate .model.build and tools for creating the ‘bare’ RES #9 is merged.
• Improve ScenarioInfo to populate sets, category sets, etc. based on the code lists.
• Remove older settings related to the year set.
• Expand documentation.

[codecov]

Codecov Report

Merging #12 (dc7b33b) into main (810e391) will decrease coverage by 0.09%.
The diff coverage is 99.77%.

❗ Current head dc7b33b differs from pull request most recent head 6c5fed5. Consider uploading reports for the commit 6c5fed5 to get more accurate results

@@             Coverage Diff             @@
##              main      #12      +/-   ##
===========================================
- Coverage   100.00%   99.90%   -0.10%     
===========================================
  Files           17       26       +9     
  Lines          647     1049     +402     
===========================================
+ Hits           647     1048     +401     
- Misses           0        1       +1     

Impacted Files	Coverage Δ
message_ix_models/testing.py	98.48% <97.22%> (-1.52%)	⬇️
message_ix_models/__init__.py	100.00% <100.00%> (ø)
message_ix_models/cli.py	100.00% <100.00%> (ø)
message_ix_models/model/bare.py	100.00% <100.00%> (ø)
message_ix_models/model/build.py	100.00% <100.00%> (ø)
message_ix_models/model/cli.py	100.00% <100.00%> (ø)
message_ix_models/model/data.py	100.00% <100.00%> (ø)
message_ix_models/model/structure.py	100.00% <100.00%> (ø)
message_ix_models/tests/model/test_bare.py	100.00% <100.00%> (ø)
message_ix_models/tests/model/test_build.py	100.00% <100.00%> (ø)
... and 22 more