FiniteAutomaton : declare a factory for intersection, etc.

[cprudhom]

Some methods are declared in FiniteAutomaton but should be moved to a factory to avoid mis-interpretation.
F-ex, calling a1.intersection(a2) returns a new FiniteAutomaton which is the result of the intersection between 'a1' and 'a2' but also modifies 'a1' (which is unexpected).