refactor: Applicative parsers

[rprospero]

This PR adds an applicative parsing library. Additionally, as a proof of concept, the ImportXYZStructure Node and ImportXYZTrajectory Node are both updates to use an applicative parser and remove the dependency on LineParser. If this code looks like a worthwhile avenue, other instances of LineParser can be converted to use the parsers.

Currently, the parser does not return error messages on failed parses, but it's possible to allow for more informative messages in the future if we change

template <typename T> using parser_output = std::optional<std::tuple<T, std::istream &>>;

to

template <typename T> using parser_output = std::variant<std::tuple<T, std::istream &>, ErrorMessage>;

As a simple tutorial on how to use the parsers, it's worth starting by looking at tests/io/applicative.cpp, as the tests build up the different combinators.

[trisyoungs]

I really like this. My only minor concern is that the expression of the parser takes a bit of getting used to with << and >> flying around everywhere. But it is cool!

A handful of suggestions sprinkled around the place, mostly flagging the usage of snake_case vs camelCase vs PascalCase.

[trisyoungs]

Love it. One added comment line.