Duplicated instances of VPhysVols are found when reading back .db files

The problem turned out to be in GeoModelRead, in the caching of the volumes that were built.

The copyNumber was used to create a cache key, together with tableID and volumeID.

But the string tableID-volID-copyN was identifying each volume uniquely... so, shared volumes were created as unique instances when restoring the GeoModel tree from file.

I fixed that by removing the copyNumber from the key used for the cache. In that way, all volumes with the same tableID and volID, but with different copyNumber, share the same Volume instance.