Add spsActor command for quick banded reads

We want to be able to request quick readouts, which requires two changes to a standard ccdActor exposure:

• add nrows=0 to the wipe command. This arranges for the voltages to be changed directly from idle to expose and for the exposureState keyword to be updated.
• add row0=N and nrows=M to the read command.

It is entirely possible that we will need to allow additional arguments.