Resize world volume functionality

This MR introduced the possibility to resize the world volume to the real need depending on the geometry that is built. This functionality is currently used in fullSimlight (and all the G4 dependent tools) when a geometry is built starting from a plugin and gmcat. In addition to that this MR:

• Changes the name of Air used for the world volume in FullSimLight/gmcat to avoid conflicts with other definitions
• Makes sure the definition of Air is the same as the one used in Athena
• Fixes a compilation bug in the HelloGeoReadG4 example