GDMLtoGM: code improvements and clean up

GeoModelTools/GDMLtoGM/GDMLInterface/atomHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/densityHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/intersectionHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/isotopeHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/materialHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/materialrefHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/physvolHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/positionHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/quadrangularHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/rotationHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/sectionHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/structureHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/subtractionHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/triangularHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/unionHandler.h

GeoModelTools/GDMLtoGM/GDMLInterface/zplaneHandler.h

GeoModelTools/GDMLtoGM/src/volumeHandler.cxx