Implemented importing regions from ROOT geometry if regions are available.
User defined regions may be exported in GDML files coming from Geant4. The master of ROOT is available to read these regions and wrap them in TGeoRegion objects. This branch changes GeanttVDetectorConstruction class to use this information if available and to create GeantV regions based on that, attaching them to vecgeom logical volumes.
This is tested to work with cms2018.gdml geometry. The merge is pending activation of ROOT master as default to compile against.