Concepts for Skills should be Separated from the NamedType

[stellarpower]

There's a nice set of concepts/traits for the skills in underlying_functionalities.hpp that would be useful for general-purpose code, including where concepts support isn't present. It would be nice to separate these out (poissibly even into a separate mini-library) from the NamedType aspect of the code, such that they can be re-used. I think also the skills would be easier to read if the concepts aspect were separated out from the NamedType-specific bits (i.e. frequent calls to .get()). I may have a go at this myself, be it as a separate small library or just integrated back here, I'll keep you posted if I do get the time.