[SCIP] correct way to fix variables locally in consprop

[Brannon King]

> However, I have an issue in that I get overconstrained (non-optimal)
> results if I call branchVar inside consenfolp...

Well, I thought it was related to branchVar, but actually, if I return
infeasible there, it also fails. It was my understanding that
consenfolp should return infeasible (or branch) when the relaxed
solution (obtained via getSolVal) was infeasible. I only expect my
constraint (modeled in the custom handler) to hold at optimal
solutions. There may be many relaxed solutions that violate it on the
way to optimum, but branching shouldn't eliminate those potential
paths. Maybe I misunderstood how to handle this scenario in
consenfolp.