Instead simply prepend "Ref/" to output ROOT file name
Cherry-picked [0adb52b6c0abe3bcdcf57db8698f09f72a84dae0](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/22/diffs?commit_id=0adb52b6c0abe3bcdcf57db8698f09f72a84dae0) as it can be singled out nicely from [!22](https://gitlab.cern.ch/evtgen/evtgen/-/merge_requests/22) to make that MR lighter.
This is related with the general thread-safety modifications: T168