[r2dbe_adc_lib/quantize] Complete change to how 2-bit quantization is done.

Instead of having a gain stage following by a quantization to 2-bit two's
complement, we instead have a threshold value that is used to determine
the 2-bits out. This is more efficient and it's the way Russ does it in
the RDBE.
[r2dbe_adc_lib/quantize] Modifed quantizer to use VDIF's offset binary encoding.
[r2dbe_adc_lib/quantize] Fixed bug causing incorrect quantization.

This was simply a wire that was accidentally connected to the wrong place.