Add darktime= argumnt to the expose readout command

Data processing needs to know the dark time of an exposure as well as the exposure time. In most cases this is essentially the same, but if we ever pause exposures the calling sequencer will need to tell the ccdActor what the readStart - wipeEnd time is.

The ccdActor's readout command now accepts an optional darktime=S argument. If not present it adds the expected shutter move time except for biases and darks. So there is no rush on this.