Add "cryoMode" keyword and command to control it.

The xcuActor is the natural program to maintain a pre-cryostat cooling/pumping mode, which is needed for the alertsActor to know what safety limits to tests against.

In the longer run, the xcuActor can track this intelligently. For now, we just want to report it. So add a command to set the mode.

Start with ['idle', 'pumpdown', 'cooldown', 'operation', 'warmup']