Relax check for whether we can build a sequence that matches a regex

[rlouf]

We can check whether a vocabulary allows to build a sequence that matches a regex when initializing Regex. The check is currently implemented as:

states_with_transition = {x[1] for x in pstate_to_vocab.keys()}
if len(self.regex_fsm.states.difference(states_with_transition)) > 0:
    raise ValueError(
        "The vocabulary does not allow us to build a sequence that matches the input regex"
    )

However this check might be too strict. While some states might be unreachable with a dictionary there could still be a path to terminal states. We should probably raise a warning when this happens, not an exception.