Support "flashing" H4s in the IDG optics lab

For n8, the optics lab cryostat, the hx ramp command accepts additional arguments to choose an LED and LED power. This turns the lamp on as soon as the 1st read starts and turns it off after the last read is done.

For characterization, we want to be able to "flash" the detector. Add a "lampReads=N" argument to specify when to turn the lamp off.