Add SimulateCavern simFlag, which can be used independently of CavernBG and cosmics simulation

As reported in ATLASSIM-4216, for radiation studies it is useful to be able to include the ATLAS Cavern in simulations without making the other configuration tweaks associated with Cosmic-ray and Cavern Background simulation. This merge request adds a SimulateCavern simFlag which is set when running Cosmic-ray and Cavern Background simulation jobs, but can also be set separately to allow the cavern to be included in simulations with no other changes.