Fix write and read of GeoFullPhysVol as root volume

This MR fixes #84 (closed).

While trying to read back a .db file in which an instance of GeoFullPhysVol was used as the Root volume, @todorova got a crash with this error message:

ERROR! A pointer to a GeoGraphNode instance is NULL {root getRootVolume [GeoPhysVol* GeoModelIO::ReadGeoModel::getRootVolume()]}. Exiting...ERROR! A pointer to a GeoGraphNode instance is NULL {root getRootVolume [GeoPhysVol* GeoModelIO::ReadGeoModel::getRootVolume()]}. Exiting...

The error came from GeoModelIO/GeoModelRead.

I investigated and found that ReadGeoModel crashed when trying to perform a dynamic cast to GeoPhysVol when retrieving the Root volume with the getRootVolume method.

All methods handling the Root volume, in fact, only handled instances of GeoPhysVol, and that was for historical reasons only: when I started to develop GeoModelIO, I initially had Root volumes as GeoPhysVol and never modified them.

But, of course, a GeoFullPhysVol can very correctly be used as the Root/World volume, so I modified the methods related to the Root to handle pointers to GeoVPhysVol: in that way, instances of both GeoPhysVol and GeoFullPhysVol can now be safely used as Root volumes.

There was also another problem in the writing. In GeoModelIO/GeoModelWrite, the method that stores an instance of GeoFullPhysVol as the Root volume in the .db file was wrongly storing the node as a GeoPhysVol: it was clearly a typo left by poor copy-paste from my side, when copying from the method handling the GeoPhysVol. This MR fixes that as well.

Also, the HelloGeoWrite and HelloGeoRead examples have been updated as well, in this MR, to reflect the above changes better.

Cc: @todorova @tsulaia @boudreau