Refactored to use templates and allows vectors to be read from settings into a std::vector container

This pull request involves refactoring the code to use templates & specialisation, instead of overloads, which reduces code duplication, and allows easier changes to be made.

The switch to a template based system was chosen to allow reading of comma separated strings into a vector (which type is determined by the user). It also allows writing of vectors to the file.

(Now with only one commit!) :)
Oh of course, I didn't realise it was only if the key was not found. That's perfectly acceptable then!