Athena switched to the external version of GeoModelXml

1. Dropped DetectorDescription/GeoModelXml from the Athena repository
2. Athena clients switched to the external version of GeoModelXml
3. In order to fix a problem related to this migration, which shows up at Geo2G4 translation, changed the way Geo2G4 determines whether it should translate a GeoModel volume into a G4 assembly. Previously the check was performed on both the material pointer and name, and now only the name comparison is done. This mechanism should probably be revisited for the long run.

CC @nstyles @rbianchi @boudreau @dellacqu