Fix heater DAC communication problems.

The 1Hz heater control loop sets the output currents of any actively controlled heaters, by reading and writing registers on the DAC8775 devices. Those reads and writes sometimes fail, after which they always misbehave, badly enough that the loop fails. Other devices (the 12 temperature sensors and the other DAC, basically) on the SPI bus are fine. Resetting the DAC, either by writing the reset register or by hitting the GPIO reset line, does not help. But restarting the program (and thus resetting the device) does.

I bet we can figure out how to reset and reinitialize, but we should find out whether the SPI traffic shows that this a program error or a hardware problem. Note that the low-level SPI control in the program is sketchy at best.