Make it possible to run MENTO when commissioning and user beamtime are present simultaneously

Fix bug where MENTO would fail to setup when a commissioning run was simultaneously running with a user beamtime. Now MENTO picks the user beamtime if it is running, and if not, chooses a commissioning run if it is present, or fails if neither is present.

This now avoids one having to manually set the root directory for MENTO, either to /gpfs/commissioning or to /gpfs/current, depending on whether a commissioning or a user beamtime was running.