Rename the internal 18-to-16 bit selection mechanisms

The FPGA can select any of the three possible 16-bit ranges in the ADC's 18-bit word. That all works, and there is a clean high-level command to choose. But the internals in the FPGA code and the calling ics_xcu_fpga code grew into this with the new ADC, and are peculiar and confusing. Should be renamed when there is leisure time.