[SCIP] correct way to fix variables locally in consprop

[Brannon King]

> SCIPfixVar() changes global bounds. To do changes that are local to the
> current node, use SCIPtightenVarLb() and SCIPtightenVarUb().

Thank you. tightenVar works for me. I have that portion working.
However, I have an issue in that I get overconstrained (non-optimal)
results if I call branchVar inside consenfolp. Is there some situation
where I should not call branchvar from consenfolp? Is it expected that
I can branch from a high-enforcement-priority constraint handler?