Restore roughActor's ability to query pressure gauges

Damnit. I forgot about the roughActor when making the INSTRM-601 changes to support old and new Pfeiffer gauges. That work factored out the command protocol from the PCM protocol, through which device the xcuActor needs to tunnel, and basically removed the ability to communicate with a gauge directly.

The roughing actor only supports a single gauge command: the get pressure. Call the handful of lower-level routines to actually send that command and parse the response.