Turn OFF mower if voltage level is critical

[MagnusPer]

When the mower doesn't find "home" or are stuck somewhere the battery will be drained. Add a third voltage limit Voltage_Limit_CRITICAL triggering to turn OFF the mower fully to not damage the battery.

Also, the comment in the defines is not fully correct, should instead be something like Voltage_Limit_LOW // When charging is needed

#define Voltage_Fully_Charged  25000  // When to consider battery fully charged, if using lithium, don't fully charge, the battery will last longer.
#define Voltage_Limit_LOW 22000  // When battery is "empty"