Override of the upstream DD4hep_GdmlDetector to provide gdmlFile name

This is a copy of create_detector in https://github.com/AIDASoft/DD4hep/blob/master/DDCore/src/gdml/GdmlPlugins.cpp but with support for name in the gdml tag to only load the volume with that name (required to be at top level).

• make gdmlFile attribute name optional for backwards compatibility