Draft: G4VUserDetectorConstruction Usage
Changes to use G4AtlasDetectorConstructionTool (which inherits from G4VUserDetectorConstruction) more like it is intended to be used by Geant4. This first set of changes simply moves the creation of the top level world volume inside this class. It also adds a dummy implementation of the ConstructSDandField().
The intention is to move the SD creation and Magnetic field creation inside this class too, but care is required to align the initialization of athena and Geant4 appropriately.