such that other user actions can access it (e.g. for adding the MCParticle collection)

    - renamed namespace tgeo to dd4hep
       -> users need to replace tgeo::mm by dd4hep::mm

 - protect against missing collections in test_surfaces

 - enforce usage of units from TGeoUnits.h in xml expression evaluators
 - convert to degrees for angles in TGeoShapes constructors
 - NB: there is one inconsistency left here:  angles returned 
       from TGeoShapes are already in degrees, this is the one
       case where a quantity returned from DD4hep/TGeo does not have the default
       units - thus one should not write
       double phi = coneSeg.Phi1() / tgeo::rad ;
       but rather
       double phi = coneSeg.Phi1() * tgeo::deg ; 
 

   for cylindrical surfaces