Revisit Public Methods of DetectorModel & Derivates

The following discussion from !632 (merged) should be addressed:

• @simonspa started a discussion: (+2 comments)

  Right now, all your methods live in the public space of the class - do you need access to all of them or could we consider moving some of them into private, just to make it more clear what is supposed to be accessed from the outside? I am especially concerned with the set* methods here, but also some of the geters might only be required for internal coordinate transforms.