Add modelinstance support

[Wegatriespython]

This is an experimental branch connected to IXMP PR adds support for the Gamsmodelinstance APIwhich provides a persistent model object for use.

Right now this is quite experimental, hence the code is not up to quality. The API for allowing user access is also not final. Putting it here for reference and feedback. For most models, model generation times dominate solving times. This means when a sequence of scenarios are to be evaluated in an ensemble a large fraction of the time is spent in the generation of the model matrix. Modelinstance provides a persistent object with an in-memory database for modifiable parameters via sync_db.

In theory this allows for gains = 1 + time_build/time_solving. For the baseline model time_build is ~500s and solve is 200s which yields a gain of 3.5X for large number of runs. Additionally the model instance passes the previous solution's basis to the next solution. In theory then a barrier followed by Simplex methods with advanced basis can approach even higher performance gains. But this requires dynamic solver parametrization maybe ML methods could help here.

How to review

At this point feedback and usage would be appreciated

PR checklist

Experimental so this will not apply till at least an early Alpha stage is reached.

- [ ] Continuous integration checks all ✅

- [ ] Add or expand tests; coverage checks both ✅
- [ ] Add, expand, or update documentation.
- [ ] Update release notes.

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[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.4%. Comparing base (12cf19e) to head (42fe0ae).
⚠️ Report is 56 commits behind head on main.

Files with missing lines	Patch %	Lines
message_ix/core.py	40.0%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #983     +/-   ##
=======================================
- Coverage   96.4%   96.4%   -0.1%     
=======================================
  Files         60      60             
  Lines       5135    5140      +5     
=======================================
+ Hits        4954    4955      +1     
- Misses       181     185      +4     

Files with missing lines	Coverage Δ
message_ix/core.py	96.0% <40.0%> (-1.5%)	⬇️

... and 1 file with indirect coverage changes

[glatterf42]

Same as iiasa/ixmp#605 (comment), I'd like to make sure that we don't create incompatibilities in our stack by having one official main version and an outdated version that we direct people to use (such as this branch). Do you know when you'll be able to continue here?