Adapt to new ADC cards

The new ADC cards appear mostly identical to the old ones. There are a couple of exceptions which we need to track. I am hoping to use the serial numbers to automatically switch between types.

• IR has opposite polarity. This will remove the need for the changes in INSTRM-486, but will require different a different clocking routine.
• the bias setting circuitry is different, and presumably simpler.
• we keep and use the sign bit from the 18-bit ADCs, so the FPGAs needs to drop the bottom two bits instead of the sign bit and the lowest bit. [ Sadly the ADCs are crappy enough that there is no gain to be had from keeping more than 16 bits. ] The FPGA already supports this, but that code may never have been tested. If we need a new FPGA image I'll make a separate ticket.