Introduce Table.remove_data()

[glatterf42]

As announced in #158, this PR adds Table.remove_data() (and fixes some type: ignore comments).

Two things to note here, mainly:

• It would be ideal to find a way to skip the data validation when removing data. As the code is written now, table.data = ... triggers the data validation, but when we're removing data, we don't actually need to validate the data remaining in the table as that has been validated when we added it previously.
  Unfortunately, sqlalchemy's docs don't hint at any possibility to skip the validation check when using @validates as we do. They do state that removals from a collection generally don't trigger this, but we don't have a proper collection, just a dict data attribute. So this would be a benefit of normalizing our data model further, since table.data would then be a proper collection.
  In the meantime, though, I was wondering whether we need @validates at all. We do need to validate data, but we don't expose table.data in the DB layer, so there's no chance of a user accidentally setting this to some invalid data. Thus, I'm not sure we benefit all that much from the @validates functionality of automatically checking .data each time it is set; we could simply make sure that add_data() always calls our validation function manually -- and conversely, that remove_data() never does that.
  What do you think?
• I'm not entirely sure why remove_data() doesn't seem to need a self.session.commit(), while add_data() does need it to pass the tests. I'm not sure we need to worry about this, either, though, as I'm happy as long as the test work as intended.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.5%. Comparing base (061a25d) to head (8eec6db).

Additional details and impacted files

@@                        Coverage Diff                         @@
##           enh/introduce-indexset-remove-data    #159   +/-   ##
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  Coverage                                88.4%   88.5%           
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  Files                                     223     223           
  Lines                                    8055    8083   +28     
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+ Hits                                     7126    7155   +29     
+ Misses                                    929     928    -1     

Files with missing lines	Coverage Δ
ixmp4/core/optimization/table.py	92.7% <100.0%> (+0.3%)	⬆️
ixmp4/data/abstract/optimization/table.py	97.0% <100.0%> (+<0.1%)	⬆️
ixmp4/data/api/optimization/equation.py	94.2% <100.0%> (ø)
ixmp4/data/api/optimization/table.py	94.6% <100.0%> (+0.6%)	⬆️
ixmp4/data/api/optimization/variable.py	94.2% <100.0%> (ø)
ixmp4/data/db/optimization/table/repository.py	98.6% <100.0%> (+0.3%)	⬆️
ixmp4/server/rest/optimization/table.py	97.2% <100.0%> (+0.3%)	⬆️

... and 1 file with indirect coverage changes