Make add virtual and add to common base so that both GeoPhysVol and GeoFullPhysVol can be used as envelopes

Follow-up from !141 (closed)

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.

FYI @boudreau @mbandier @dellacqu

GeoModelCore/GeoModelKernel/GeoModelKernel/GeoFullPhysVol.h

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef GEOMODELKERNEL_GEOFULLPHYSVOL_H
@@ -32,8 +32,8 @@ class GeoFullPhysVol final : public GeoVFullPhysVol
  public:
   GeoFullPhysVol (const GeoLogVol* LogVol);
 
-  /// Adds a Graph Node to the Geometry Graph.
-  void add(GeoGraphNode* graphNode);
+    /// Adds a Graph Node to the Geometry Graph
+  virtual void add(GeoGraphNode* graphNode) override final;
 
   /// Returns the number of child physical volumes.
   virtual unsigned int getNChildVols() const override;