fix: fix bug in scip freeConstraints and gurobi setConstraints crash

[tlambert03]

This fixes the SCIP freeConstraints no-op bug... but there is still a crash in the gurobi backend (that should appear in the tests now) ... and bug in gurobi clear is also fixed

[codecov]

Codecov Report

Merging #44 (5d3ec1b) into main (d1fb38d) will decrease coverage by 25.97%.
Report is 1 commits behind head on main.
The diff coverage is 23.52%.

@@             Coverage Diff             @@
##             main      #44       +/-   ##
===========================================
- Coverage   91.90%   65.93%   -25.97%     
===========================================
  Files           4       17       +13     
  Lines         383      822      +439     
  Branches        0      155      +155     
===========================================
+ Hits          352      542      +190     
- Misses         31      219      +188     
- Partials        0       61       +61     

Files	Coverage Δ
ilpy/impl/solvers/GurobiBackend.h	0.00% <ø> (ø)
ilpy/impl/solvers/ScipBackend.cpp	60.52% <50.00%> (ø)
ilpy/impl/solvers/GurobiBackend.cpp	9.02% <0.00%> (ø)

... and 11 files with indirect coverage changes

[funkey]

Thanks a lot! I think there is a potential memory leak in ScipBackend, otherwise looks good to me.