also makes protected fields private

Closes #205

Closes #215

This new easy algorithm checks whether a regexp's language is an
infinite language.
Also adding an AQL test to compare with the same algorithm on finite
automata.

Introduce a new algorithm, regexp::properties::LanguageIsEpsilon to
check whether a language of a regexp is exactly equal to a language of
epsilon regexp.
This will be later useful in checking whether the language of a regexp
is infinite.

Similarly to previous commit, let's make this clear. We are soon going
to add a new algorithm that will check if the language is equal to
epsilon, so let's differentiate.

The name regexp::properties::RegExpEmpty is not that expressive. Let's
make clear what is going on.