AC-FOVE: code refactoring for SimpleParfactor (expansion)

* StdParfactor: expand() method
* Tuple: static factory to copy tuples
* Bool, RangeElement, Histogram: combine() method. Useful for
Histograms only
* CountingFormula: remove(), takeOut(), simplify(). Auxiliary methods
to make expansion code more legible
* StdParfactorTest: test to expansion operation