Remove Column class entirely

[glatterf42]

As discovered in #151, using sqlalchemy's associationproxies is both (a bit) more performant and (considerably) less confusing than having a DB table called Column. Thus, this PR completes the removal of this construct.

TODO

• DB migrations, though ideally, Remove Column class for Parameter #156 gets its own migrations first

On top of this, the PR includes a few minor fixes:

• Make type hints of .data in the core layer more accurate
• Fix some type: ignores by casting appropriately
• Fix some docstrings wrt the errors they raise
• Make Variable.indexsets None when appropriate
• Fix missing .set_creation_info() calls
• Skip data validation iif appropriate (we didn't skip checks for e.g. variable.data with only "levels" and "marginals" after Introduce run.optimization.remove_solution() #139 and never skipped for tables, even when {} was added)

[glatterf42]

I changed this to reflect the other changes, then run into the thing that the tests on the REST API platforms always converted data to dict[str, list[float]] for some reason. I don't understand why, but it doesn't seem to be this change because the same happens for equations. Still, to show that, I didn't also change the other type hints in this layer, though I suppose I still could.

[codecov]

Codecov Report

Attention: Patch coverage is 98.83721% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.4%. Comparing base (0733b76) to head (9c36f7a).

Files with missing lines	Patch %	Lines
ixmp4/data/db/optimization/parameter/model.py	85.7%	1 Missing ⚠️
ixmp4/data/db/optimization/table/model.py	95.2%	1 Missing ⚠️

Additional details and impacted files

@@                     Coverage Diff                      @@
##           enh/introduce-has-solution    #157     +/-   ##
============================================================
- Coverage                        88.4%   88.4%   -0.1%     
============================================================
  Files                             231     223      -8     
  Lines                            8113    8032     -81     
============================================================
- Hits                             7179    7103     -76     
+ Misses                            934     929      -5     

Files with missing lines	Coverage Δ
ixmp4/core/optimization/equation.py	93.3% <100.0%> (-0.3%)	⬇️
ixmp4/core/optimization/parameter.py	93.0% <100.0%> (-0.2%)	⬇️
ixmp4/core/optimization/table.py	92.4% <100.0%> (-0.2%)	⬇️
ixmp4/core/optimization/variable.py	93.4% <100.0%> (-0.3%)	⬇️
ixmp4/data/abstract/optimization/__init__.py	100.0% <ø> (ø)
ixmp4/data/abstract/optimization/equation.py	97.0% <100.0%> (+<0.1%)	⬆️
ixmp4/data/abstract/optimization/table.py	96.9% <100.0%> (+<0.1%)	⬆️
ixmp4/data/abstract/optimization/variable.py	97.0% <100.0%> (+<0.1%)	⬆️
ixmp4/data/api/optimization/equation.py	94.2% <100.0%> (ø)
ixmp4/data/api/optimization/table.py	94.0% <100.0%> (ø)
... and 18 more