Avoid superfluous DB calls from facade

[glatterf42]

While working on #148, @meksor and I noted that there were always two calls to validate_data when adding data to a Parameter. This is superfluous, of course, and the culprit is in the core layer: in parameter.add(), we add_data(), but then also set parameter._model.data, which triggers another validate_data() run because that always fires when the attribute is set.

This PR fixes that by setting parameter._model instead of just .data, also for all other relevant optimization items. This significantly boosts their performance.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.9%. Comparing base (0deac5b) to head (007950e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #149     +/-   ##
=======================================
- Coverage   86.9%   86.9%   -0.1%     
=======================================
  Files        230     230             
  Lines       8156    8156             
=======================================
- Hits        7095    7094      -1     
- Misses      1061    1062      +1     

Files with missing lines	Coverage Δ
ixmp4/core/optimization/equation.py	93.5% <100.0%> (ø)
ixmp4/core/optimization/indexset.py	91.5% <100.0%> (ø)
ixmp4/core/optimization/parameter.py	93.3% <100.0%> (ø)
ixmp4/core/optimization/table.py	92.5% <100.0%> (ø)
ixmp4/core/optimization/variable.py	93.6% <100.0%> (ø)

... and 1 file with indirect coverage changes

[danielhuppmann]

Nice! Thank you!

I now remember that I noticed this issue a while back in the tests but didn't manage to solve it at the time...