Possibly add minimum opening pressure override.

It might be sensible to add a mechanism to allow opening the gatevalve when there is some small allowable pressure inside the cryostat. As it stands, below some fairly high threshold (variable, default = 500 mbar), we do not believe the interlock transducer pressures and therefore block opening. This can be inconvenient when there is a very small pressure (< 1Torr, say, but too high for the transducers to read 0) in the cryostat: we now always need to backfill to atmosphere.