Extend Match-Tree to deal with multivalued variables

[gfrances]

Meaning that it would support a small but nonetheless useful fragment of FSTRIPS.
Namely, only problems where the preconditions of all actions is a conjunction of FOL equality atoms involving at most one state variable, i.e. of the form X=c, would be supported.
We could actually lift the restriction of atoms being equality atoms and give support to atoms such as X<c, etc., but I'd leave that for a future implementation.