Introduce Common.ProductionStep flag and migrate sim & overlay workflows

This is a first step in cleaning up detector flags in CA-based config.

Introduce a new Common.ProductionStep flag and ProductionStep enum which will replace the majority of the use of Detector.Overlay*, Detector.Digitize* and Detector.Simulate* flags. For now migrate the simulation and overlay parts.

AtlasGeometryCommon/BeamPipeGeoModel/test/BeamPipeGMConfig_test.py

 #!/usr/bin/env python
 """Run tests on BeamPipeGeoModel configuration
 
-Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
+Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 """
 if __name__ == "__main__":
     from AthenaCommon.Configurable import Configurable
@@ -10,9 +10,6 @@ if __name__ == "__main__":
     from AthenaConfiguration.TestDefaults import defaultTestFiles
 
     ConfigFlags.Input.Files = defaultTestFiles.HITS
-    ConfigFlags.Detector.SimulatePixel = False
-    ConfigFlags.Detector.SimulateSCT   = False
-    ConfigFlags.Detector.SimulateTRT   = False
     ConfigFlags.GeoModel.Align.Dynamic    = False
     ConfigFlags.lock()