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 87.0%. Comparing base (0deac5b) to head (b0201c6).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #139   +/-   ##
=====================================
  Coverage   86.9%   87.0%           
=====================================
  Files        230     230           
  Lines       8156    8162    +6     
=====================================
+ Hits        7095    7101    +6     
  Misses      1061    1061           

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