- changed default unit for angles to be radians (tgeo::rad = 1.)

 - 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 ;