Harmonize the battery level ramp

[BaptisteHudyma]

The battery level of a Liion is not linear, and currently we have a dumb way to convert from the voltage of the liion to the actual battery level (simple 3 curve approximation).
This level is remaped to the "safe" battery level (the max and min voltage for the battery, defined by the user).
(Check the battery.cpp file)

This method is imprecise, and breaks occasionally the logic of the application (eg: low battery shutdown).
It should be replaced by a finer liion discharge curve.

Suggestions:

• All liion level to battery level values could be stored in a look up table