Allow GeoModelXml envelopes to be GeoFullPhysVol as well as GeoPhysVol

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.

The latter does not inherit from the former, nor do they share a common base class with a virtual add method, and so we need to treat each case separately rather than a more simple solution that could support both through polymorphism. That being said, there could well be a better way to do this than what I've implemented here.

FYI @mbandier @dellacqu in case you have comments or suggestions.