This is the DFA class, defining a Flexc++'s dfa matrix. One DFA matrix is
built for every start state.

The constructor receives the Rules and a vector of rule-numbers to use.

The initial start condition is determined, considing of the start states of
the DFA's rules.

 the eClosure (ASU, par. 3.6,
p. 118) of the initial states of the active rules.
