Tracker geom det

Following the presentation at the ALCA meeting about APE for muons
https://indico.cern.ch/event/294021/contribution/1/material/slides/0.pdf
It became clear that the GeomDet inheritance tree shall be split between Muon and Tracker at root level.
This requires the coalescence of GeomDetUnit into GeomDet and the introduction of two root classes
TrackerGeomDet and MuonGeomDet.
This PR prepares the infrastructure and implements the Tracker components, leaving to the Muon POG to take care of their specific parts (and if necessary to generalize and extend some of the APE components).

It is quite obvious at this point that the Tracking Geometry infrastructure, as designed in the early days, is not able anymore to support new requirements (it hardly supported alignment requirements even when designed).
Most probably a general clean-up would be very welcome. Unfortunately the amount of code to modify is, as usual, enormous and residing mostly in non critical areas. So I suspect we have just to live with it.