add extra resolution (nV) to the signal chain

We were seeing quantization of the data which turned out to be due to rounding of intermediate
results to 6 decimal places when they were passed between scripts.  We extended this to 9
decimal places, which seems to have fixed the problems with quantization.
change gain in frame-parser to 24

The gain setting of 0b110 corresponds to 12x on the ADS1298, but 24x on the ADS1299.
Because we're currently using the 1299, 24x is the correct multiplier to use.  In the
long run the firmware should detect which chip is in use and report the correct multiplier.