AP_HAL: added voltage_average() interface to AnalogIn

returns voltage in Volts, using averaged reading over samples.

Where possible this should be auto-scaled against a known reference
voltage
AP_ADC_AnalogSource: added voltage_average() interface
Plane: use voltage_average() to not depend on INPUT_VOLTS
Copter: use voltage_average() so as to avoid INPUT_VOLTS
Rover: use voltage_average() interface