Introduce Indexset.remove_data()

[glatterf42]

For iiasa/ixmp#557, I will next need functions that allow removal of individual keys from Indexsets, Tables, and Parameters. This PR adds such a function for Indexsets.

While working on that function, I found myself thinking: wouldn't it be nice to get some sort of warning if you're trying to remove data from an Indexset, but that Indexset doesn't have that data? This can be checked easily in sqlalchemy, so I added log.info() messages for two cases.
In order to test that these messages are actually logged, I copied over the assert_logs() utility function we have in ixmp and enhanced it with some type hints. Thanks to @khaeru for writing that in the first place!

Note

This PR looks larger than it is because I swapped the order of the tests in the data/ subfolder to be consistent with the other optimization tests. There's no need to verify test_list_indexset and test_tabulate_indexset again.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.4%. Comparing base (9c36f7a) to head (061a25d).

Additional details and impacted files

@@                     Coverage Diff                      @@
##           remove-column-class-entirely    #158   +/-   ##
============================================================
  Coverage                          88.4%   88.4%           
============================================================
  Files                               223     223           
  Lines                              8032    8055   +23     
============================================================
+ Hits                               7103    7126   +23     
  Misses                              929     929           

Files with missing lines	Coverage Δ
ixmp4/core/optimization/indexset.py	91.9% <100.0%> (+0.4%)	⬆️
ixmp4/data/abstract/optimization/indexset.py	96.4% <100.0%> (+0.1%)	⬆️
ixmp4/data/api/optimization/indexset.py	97.7% <100.0%> (+0.1%)	⬆️
ixmp4/data/db/optimization/indexset/repository.py	98.4% <100.0%> (+0.3%)	⬆️
ixmp4/server/rest/optimization/indexset.py	100.0% <100.0%> (ø)