GDML use offline

[Laurie Nevay]

GDML loading requires online access to the GDML schema. This should fallback to a local copy by default.

[Laurie Nevay]

The schema is now included by default. The location is specified by the tag at the beginning of the document. Given our new preparsing and editing capability of xml files using xercesc before using the Geant4 GDML parser, we can edit this to be a location we choose.

As it's not the easiest thing to test if a computer has an internet connection, we default to the provided one in BDISM (onus on us to update this) or if the user explicitly specifies one (ie not an url), that will be left.