Fix the union operatior on parallelotopes.
A bug in the pivoting procedure in the compation object prevented union to work in
many cases.
Modularize implementation of SumDomain.
Now there is SumDomain which is an abstract class for all sum domains,
SumGenericDomain which is a generic implementations of SumDomain without
heuristics, and SumIntParallelotopeDomain which is for Int+Ptopes with
corresponding heuristics.