Improve battery discharge reporting

[jonnew]

A quick test seems to indicate that there is a very loose relationship between the display battery level and the amount of useable battery life remaining. This is not surprising given that these constants:

ephys-test-board/firmware/battmon.c

Line 7 in f8f7393

#define BATT_VOLTAGE_LOW 3.6f // Low voltage of linear discharge range

were chosen without much thought. The discharge curve for the device should be calculated and used as a lookup table to calculate remaining battery life.