Improve MCS exposure times

The time taken to take an MCS exposure is typically 15s. The majority of this is overhead (the integration time is 0.8s). Please look into ways to reduce this.

Comment from RHL:

I understand that some of this is due to the CANON driver, so the minimum time is 3*0.8 == 2.4s, but I think that the rest of this is under our control. In the slightly longer term, I suspect that we could leave the camera running all the time (well, not during 1200s SpS exposures) and discard images that we don't want. That should avoid the 1.6s CANON overhead, but may or may not be worth the trouble.