Add addExprNonlinear

[Joao-Dionisio]

This is not the best way to solve the issue, but I think it gets the work done.

• SCIPaddExprNonlinear requires a nonlinear constraint, but it should be possible to convert a linear constraint into a nonlinear one in scip.pxi (x = self.addVar(lb=0,ub=0), self.addCons(linear expr + x**2 <= RHS)). This would work with other methods.
• I'm using a Python union, since GenExpr doesn't include Expr. Most likely it should.
• I couldn't find a proper way to get a scip expression from the python expression, so I add a temporary constraint, and then delete it.

[Opt-Mucca]

I feel like I'm playing the opposition on these merge requests......

In what context does something like this come up @Joao-Dionisio ? I'm not against the change, but it feels incredibly niche. Couldn't you simply just remove the constraint and then re add it with the now modified expression? (Adding a linear term to the non-linear expression should work fine)

[Joao-Dionisio]

No worries @Opt-Mucca, I think it was me who spammed PRs of dubious interest :)

This came up with Issue #713, and it seems like it could be helpful, maybe for reoptimizations and such. I suppose it would serve the same purpose that it serves in SCIP.

[Opt-Mucca]

@Joao-Dionisio I'm convinced for this one after seeing the issue you commented. I still think you could simply keep track of the expression inside of the constraint, but that is actually duplicate work, and maybe not that intuitive.

Suggestion: Do you need to add coef * expr instead of just the expression? The user could always just put coef * expr as the input then.

[Joao-Dionisio]

@Opt-Mucca yeah, you just need the expression, but I tried to keep the function as close to the SCIP function as I could, which does take coef as an input.

[Opt-Mucca]

Then I'm happy with this one!

[Joao-Dionisio]

Oh man, what have I done... I just wanted to add a TODO :(

Anyway, we talked about accepting this if we added a TODO. Since it seems a bit annoying to actually solve the problems, and they're not that big, then I say we do this. It also frees up an issue.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 52.76%. Comparing base (5a166be) to head (676939d).

Files	Patch %	Lines
src/pyscipopt/scip.pxi	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #760      +/-   ##
==========================================
+ Coverage   52.69%   52.76%   +0.07%     
==========================================
  Files          17       17              
  Lines        3809     3817       +8     
==========================================
+ Hits         2007     2014       +7     
- Misses       1802     1803       +1     

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