Fix reading of world volume's metadata when the World is a FullPhysVol
This MR fixes a bug observed when reading back a .db file where the world volume is a FullPhysVol, by fixing a behaviour coming from the old days, when PhysVol was supposed to be a GeoPhysVol only.
It also introduces a check on the type of the world volume, just to be sure to print a meaningful error message when the wrong type of world volume is found, which, at this point, should only be possible if a .db file has been hacked by hand.
A note: this MR fixes the issue found by @todorova