FiniteAutomaton : declare a factory for intersection, etc.

[cprudhom]

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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).