G4AtlasUserActionConfig: Fix incorrect import for stopping particle simulation
ISF_ToolsConfig.py and G4CosmicFilterConfig.py both implemented a Cfg function with the same name.
The incorrect one was imported by G4AtlasUserActionConfig.py. This MR fixes this and renames the method
in ISF_ToolsConfig.py although this one will probably turn out to be obsolete and can be removed in a
follow-up MR.