Introduce optimization item delete()

[glatterf42]

One more piece we need to complete functionality also offered by ixmp: this PR adds delete() functions for all optimization items as a whole.

I tried to achieve this using sqlalchemy's cascades, which are recommended as the higher-performing option in their docs (compared to loading all elements from collections before delete()ing them piecewise).

Note

While the docs say this requires special setup for SQLite, we already have this exact setup, so can employ this immediately.

With the current setup, IndexSets cannot be deleted if they are used by some other object. IIRC, this is a requirement that @danielhuppmann mentioned at some point, which we seem to get almost for free due to our setup: the IndexSet.id columns in the e.g. TableIndexSetAssociation cannot be NULL; thus, when one tries to delete an IndexSet while it is linked to a Table (i.e. recorded in TableIndexSetAssociation), an IntegrityError/DeletionPrevented will be raised.

All other optimization items should also theoretically be able to raise DeletionPrevented, but in practice, no other item relies on their ids not being NULL somewhere, so I'm not sure how else to check this. This is the only kind of TODO mark appearing inline.
However, I'm also not sure we need to check this as the same kind of behavior is checked by the IndexSet tests or may not be relevant for the other objects.

TODO

• This PR changes some of the db/model files, but only relationship and foreignkey setup, so I'm not sure it needs its own migration files. But check if it does!

[codecov]

Codecov Report

Attention: Patch coverage is 98.86364% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.6%. Comparing base (7ca6c9d) to head (2895604).

Files with missing lines	Patch %	Lines
ixmp4/core/optimization/base.py	88.8%	1 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                        @@
##           fix/column-names-property-type    #163     +/-   ##
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+ Coverage                            88.5%   88.6%   +0.1%     
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  Files                                 223     223             
  Lines                                8117    8192     +75     
================================================================
+ Hits                                 7189    7264     +75     
  Misses                                928     928             

Files with missing lines	Coverage Δ
ixmp4/core/optimization/equation.py	93.3% <100.0%> (ø)
ixmp4/core/optimization/indexset.py	91.9% <100.0%> (ø)
ixmp4/core/optimization/parameter.py	93.3% <100.0%> (ø)
ixmp4/core/optimization/scalar.py	93.8% <100.0%> (ø)
ixmp4/core/optimization/table.py	92.7% <100.0%> (ø)
ixmp4/core/optimization/variable.py	93.4% <100.0%> (ø)
ixmp4/core/run.py	98.1% <ø> (ø)
ixmp4/data/abstract/optimization/base.py	100.0% <100.0%> (ø)
ixmp4/data/abstract/optimization/equation.py	97.1% <100.0%> (+<0.1%)	⬆️
ixmp4/data/abstract/optimization/indexset.py	96.5% <100.0%> (+0.1%)	⬆️
... and 31 more