AC-FOVE: code refactoring for SimpleParfactor (multiplication)

* StdParfactor: multiplication, new methods for builder, correction on
calculation of size(), refactoring of isExpandable()
* Parfactor: auxiliary method to multiplication
* Prv, StdPrv, CountingFormula: new method boundVariable()
* LogicalVariable, StdLogicalVariable: excluded set, size of
population satisfying a set of constraints, isEmpty()
* StdParfactorTest: test for multiplication