Add safety band heater process to SMB.

The temperature board is responsible for trying to keep the detectors warmer than some other things in the cryostat. We have evidence that safe temperature headroom is passively maintained if all power is cut off, but if there is any power available the SMB should still put in some heat.

We must assume that the network is gone and that both the site power and the instrument UPS have failed, so we should be power-conscious.

The proposed scheme is for the heater control loop to switch to a power-efficient bang-bang mode under some condition, not yet nailed down.