Export geometry in GDML format.
Added the possibility for the GeometryBuilderGeant4 and GeometryBuilderTGeo to export the geometry in GDML format. Please note that the export will not work with the current versions of Geant4 (->10.3.01), as the conversion of parametrized boolean solids is currently not supported (problems with 'bumps'). A patch is being developed and will be submitted soon.