support non-convex quadratic objectives/constraints in gurobi

[tlambert03]

closes #27

see https://www.gurobi.com/documentation/10.0/refman/nonconvex.html for relevant gurobi docs

@funkey ... while this "fixes" a crash out to the terminal with non-convex quadratic objectives, i'm not sure if we should be applying it all the time like this. let me know if you want to guard this behind some opt-in parameter setting method

edit: the crash is now fixed anyway with 037283e ... so this just adds potential support for non-convex solving now

[funkey]

I think this is fine. Can't imagine a case where someone deliberately builds a non-convex objective and doesn't want it to be solved. My guess is that Gurobi's default is for backwards compatibility, where some code might rely on Gurobi complaining if the objective is non-convex.