Latch lamps keyword when turned on

Currently, pfilamps only latches lamp keywords (state, onTime, offTime) when lamps are turned off. This works for most cases, but fails for HgCd exposures where the lamp remains on while several exposures are taken.

In the HgCd case, the lamp is not turned off between exposures, so the lamp keyword is never generated. As a result, spsActor incorrectly interprets the lamp state during the exposure.
So I had to disable that feature for all lamps, giving up on setting pfsConfig.fiberStatus correctly for science fibers.

pfilamps should instead latch lamp keywords at the moment the lamp is turned on, and make that information available for all subsequent exposures until the lamp is turned off.