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 trying to do 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)
• Apply naming convention to classes

HOWEVER, given the magnitude of the changes I'd need to apply to cover all the above, I've decided to split this effort in multiple MRs. In here I'm simply moving the files into the detail folder, adding only two unit tests and adding the detail namespace in almost all the places.

@adye @goetz