Gain values are specified with enumeration values that can't be used
to reverse the effects of scaling the input signal.  Provide a
function that reverses the effect of the gain by scaling a measured
value.

Also provide a function that converts a raw measurement captured with
a reference voltage and specific gain and resolution to the
corresponding voltage in millivolts.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>