Epsilon handling

[jurajsic]

We need some unified handling of epsilon transitions given as arguments.
Most functions now take the first epsilon symbol and all the symbols larger than that are assumed as epsilon.
Sometimes we do not want to handle these as epsilon (see for example #527), or we might want to have nonsequential epsilon symbols.
In #527 we now have std::optional<mata::Symbol> first_epsilon to represent the case where I want to print epsilons in the word, we might want to use this in other places too. Or we might want something completely different, we should decide.