Make Equation.indexsets optional

[glatterf42]

Working on the ixmp4 shim, I discovered that our assumption that Equations must always be constrained to IndexSets is wrong: in message_ix, there are numerous examples of Equations not linked to IndexSets.

This PR includes some minor improvements to the code, but mainly makes the Equation.indexsets attribute optional.

TODO

• I'm not sure the changes here need their own DB migration. It looks like only an ORM attribute/property is affected, but make sure that's true; otherwise, include the migrations.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.6%. Comparing base (2895604) to head (f907393).

Additional details and impacted files

@@                          Coverage Diff                           @@
##           enh/introduce-optimization-item-delete    #164   +/-   ##
======================================================================
  Coverage                                    88.6%   88.6%           
======================================================================
  Files                                         223     223           
  Lines                                        8192    8200    +8     
======================================================================
+ Hits                                         7264    7272    +8     
  Misses                                        928     928           

Files with missing lines	Coverage Δ
ixmp4/core/optimization/equation.py	93.3% <100.0%> (ø)
ixmp4/data/abstract/optimization/equation.py	97.1% <100.0%> (ø)
ixmp4/data/api/optimization/equation.py	94.4% <100.0%> (ø)
ixmp4/data/db/optimization/equation/model.py	100.0% <100.0%> (ø)
ixmp4/data/db/optimization/equation/repository.py	98.8% <100.0%> (+0.1%)	⬆️
ixmp4/data/db/optimization/parameter/repository.py	96.8% <100.0%> (ø)
ixmp4/data/db/optimization/table/repository.py	98.7% <100.0%> (ø)
ixmp4/data/db/optimization/variable/repository.py	98.8% <100.0%> (ø)
ixmp4/data/db/region/repository.py	100.0% <100.0%> (ø)
ixmp4/server/rest/optimization/equation.py	97.5% <100.0%> (ø)