Add alt. signature to Constraint* constructors to ligthen instantiation code (pyfaut and matfaust).

Example got matfaust:
Before it was necessary to write something like: fact_cons{1} = ConstraintInt(ConstraintName(ConstraintName.SPLIN), 500, 32, 5);
Now it's easier with equivalent code: fact_cons{1} = ConstraintInt('splin', 500, 32, 5);
The former code is still valid though.