AC-FOVE: refactoring for SimpleParfactor (counting)

* StdParfactor: counting algorithm
* Parfactor: complemented documentation for count() and expand()
* CountingFormula: range is now List<Histogram<RangeElement>> instead
of List<Histogram<? extends RangeElement>>. Less flexible, but
simpler.
* StdParfactorTest: tests for counting