add switchOn switchOff command in pfilamps pi

in the context of HgCd, as the lamp takes a fair amount to warmup, we think it'd be more efficient to keep the light on if we take back to back exposure.
We can, sort of hack this for now, by requesting a long time(calculated from exptime and overhead).
But it'd be cleaner to be able to just turn on / turn off the lamp directly from the pi.