Introduce run.optimization.remove_solution()

[glatterf42]

This PR is the first part of cleaning up #101. It introduces the run.optimization.remove_solution() in the core layer, which removes all data of all equations and variables belonging to the run.
It also includes various small clean-ups that arose from the reviews of #101.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.9%. Comparing base (c243743) to head (b145b63).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #139   +/-   ##
=====================================
  Coverage   86.9%   86.9%           
=====================================
  Files        230     230           
  Lines       8156    8162    +6     
=====================================
+ Hits        7094    7100    +6     
  Misses      1062    1062           

Files with missing lines	Coverage Δ
ixmp4/core/optimization/data.py	100.0% <100.0%> (ø)
ixmp4/core/optimization/equation.py	93.5% <ø> (ø)
ixmp4/core/optimization/parameter.py	93.3% <ø> (ø)
ixmp4/core/optimization/table.py	92.5% <ø> (ø)
ixmp4/core/optimization/variable.py	93.6% <ø> (ø)
ixmp4/data/db/optimization/equation/model.py	100.0% <100.0%> (ø)
ixmp4/data/db/optimization/equation/repository.py	98.7% <100.0%> (ø)
ixmp4/data/db/optimization/indexset/repository.py	98.1% <ø> (-0.1%)	⬇️
ixmp4/data/db/optimization/scalar/repository.py	98.2% <100.0%> (-0.1%)	⬇️
ixmp4/data/db/optimization/table/repository.py	98.4% <ø> (-0.1%)	⬇️
... and 3 more

[meksor]

A few suggestions but good all in all.

[meksor]

Suggested change

	if data_to_validate != {}:
	if data_to_validate:

or more explicit:

Suggested change

	if data_to_validate != {}:
	if bool(data_to_validate):

ditto above in the same function.
Why? Because None != {} is True.

[glatterf42]

I have to admit: I don't know how data_to_validate could be None, but I added these suggestions and will merge the PR :)