When investigating the possibility to use the existing GeoModelXml "Envelope" mechanism to insert new service material inside an existing volume (in this case to position ITk/HGTD service material in the Calorimeter crack region) it was noticed that the mechanism only works for GeoPhysVols while the specific location where new items would ideally be inserted is a GeoFullPhysVol. This MR adds the capability to also use a GeoFullPhysVol as an Envelope, through making the add(GeoGraphNode* graphNode) method a virtual method of the common base class used by both GeoPhysVol and GeoFullPhysVol, i.e. GeoVPhysVol.
