Draft: Move class to detail folder and separate in proper files

Trying to improve the readability and testing of the acts packages. In this case I'm doing the following:

• Move classed in the detail namespace into a detail folder within the src folder, hiding the implementation details from the main files
• Separate classes into multiple files to ease the readability (.h, .icc and .cxx)
• Prefer not to have multiple classes definitions into same file, to improve readability and clarity
• Will add unit tests for all these classes (separation into multiple files will help as well)

I'm also planning of adding unit tests for all these classes.

@adye @goetz would you agree on these points? I've made a simple example with one class, but I'd prefer to have your feedbacks before committing more effort into this