Coulomb counting for SOC estimation

[trebula]

Uses the existing wattHoursUsed variable to continually update the estimate of SOC.
At startup, analyze the initial SOC using the getBatteryPercent function. Once this period ends, it uses the initial SOC estimation and subtracts the wattHoursUsed to determine the new SOC. Will likely overestimate the battery percent since the actual battery capacity will probably be lower than the nominal capacity due to heat loss, but doesn't rely as much on voltage (which may be inaccurate due to the flat curve of Li-on batteries)

[zjwhitehead]

Closing this due to the latest SP140 hardware supporting BMS based SOC data directly for maximum accuracy