check interlock when enabling

[jordens]

Work around sinara-hw/Booster#369:

Before enabling a channel, check the three power measurements (input, output, reflected) and compare them with the thresholds (DAC and software threshold for the input power). If they are already within 5% of full scale (to account for calibration error), don't allow enabling the channel.
Otherwise the interlocks will fail to trip.

[jordens]

If the user has hacked their firmware to adress #117 that makes the approach outlined above a bit more tricky.