Introduce Run.optimization.has_solution()

[glatterf42]

For compliance with ixmp's API, we need a way to check whether a Run has a solution. This PR adds such a function and tests its behaviour.

I wondered a bit about the best way to do this. Theoretically, we could also add a field has_solution to the DB model of a Run and set it to True if some kind of run.optimization.solve() function is called. We don't have such a function right now, though, so I opted for the way we can already check it (which is identical to how ixmp_source does it): iterate over all variables and equations defined in the Run, and return True if any of them contain levels (i.e. some data); return False otherwise.
Of course, this could be slow if we need to iterate through millions of equations; maybe @danielhuppmann can share how many objects we'd typically have to iterate over.

For now, I've included this function in the optimization subobject since it seems similar to remove_solution() and only affects the "solving" part of a Run. However, we could also shift this up to enable run.has_solution() as this is a property of the whole Run.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.4%. Comparing base (71c0e69) to head (0733b76).

Additional details and impacted files

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##           enh/expose-run-is-default    #154   +/-   ##
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  Coverage                       88.4%   88.4%           
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  Files                            231     231           
  Lines                           8103    8113   +10     
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+ Hits                            7169    7179   +10     
  Misses                           934     934           

Files with missing lines	Coverage Δ
ixmp4/core/optimization/data.py	100.0% <100.0%> (ø)
ixmp4/data/db/optimization/equation/model.py	100.0% <100.0%> (ø)
ixmp4/data/db/optimization/variable/model.py	100.0% <100.0%> (ø)