Add .models.shift_period()

[khaeru]

This PR adds a function shift_period() that implements, purely in Python, the behaviour of Scenario.clone(…, shift_first_model_year). Because the latter is currently implemented in the Java code underlying ixmp.JDBCBackend, it cannot be extended in parallel with changes to the implementation of MESSAGE in GAMS (or, in the future, other implementations).

• Partly addresses Disentangle message_ix and ixmp_source #254.
• Also due to limitations of ixmp.JDBCBackend, is it not supported to set values on model data items of type "variable" or "equation" directly. Thus, this PR can't be completed without Implement IXMP4Backend ixmp#516, Add Python DB API backend ixmp#400, or similar.
• cf. also Implement clone() in Python ixmp#424. While the behaviour is currently provided as an argument to the clone() command, the new function is distinct.
• This PR does not directly address, but will make it much easier to address:
  • Add parameter historical_total_capacity #542
  • Populate historical_emission automatically when shifting y₀ #543
  • Extend historical parameters to track all VAR when shifting firstmodelyear #867
• The operation of shifting the first model period forward in time is lossy, because parameters like historical_activity have fewer dimensions than the corresponding variables (e.g. ACT). For the opposite operation—shifting the first model period backwards in time—essentially the scenario must be solved again to generate the corresponding solution data in e.g. ACT.

More to be added

How to review

To be added

PR checklist

• Complete the implementation
  • Invoke shift_period() from message_ix.Scenario.clone(…) if the corresponding argument is given.
• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.6%. Comparing base (175d64d) to head (fea126e).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #873   +/-   ##
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  Coverage   95.6%   95.6%           
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  Files         46      46           
  Lines       4344    4365   +21     
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+ Hits        4154    4175   +21     
  Misses       190     190           

Files	Coverage Δ
message_ix/models.py	99.1% <100.0%> (+<0.1%)	⬆️
message_ix/tests/test_models.py	100.0% <100.0%> (ø)