WIP: Factorize GeometryBuilder

In view of !73 (closed) I have started to restructure and factorize the geometry builder a bit to provide some re-usable snippets of code which can be used for different detector models.

This is rather straight forward due to the get/setExternalObject() routines of our detector implementation, which allows to create and store the different parts of the detector geometry independently.

@kwolters I would like to receive some ideas from your side on how to proceed. In the end, I would like to separate the building of different detector types (or: appliance types) into their own files for convenience, however, many things are re-usable and can be handled centrally. Would you go ahead implementing a fully specialized template function for the different models? Or do you have a different idea?

This fixes #71 (closed)