[SCIP] correct way to fix variables locally in consprop

[Brannon King]

I'm writing a custom constraint handler. I started with using fixvar
in both consprop and consenfolp. This runs to completion, but the
problem never reaches the correct minimum. It seems to be
overconstrained. One theory I had about this was that fixvar runs at a
global level (true?); I need consenfolp to fix variables at the node
level, not globally. However, when I change consenfolp to use
chgVarLbNode instead of fixvar, I get infinite calls on consprop. What
is the correct mechanism to fix variables (locally) in consprop and
consenfolp? How do I avoid consenfolp triggering consprop in all
situations?