AP_HAL: added voltage_average_ratiometric() call to AnalogIn

this is for ratiometric sensors such as the 3DR airspeed sensor and
the Maxbotix analog sonar
AP_Airspeed: use rationmetric analog input for airspeed sensor
AP_Rangefinder: use ratiometric analog input for analog sonar
AP_ADC_AnalogSource: added voltage_average_ratiometric()
Rover: use voltage_average() for RSSI

this allows it to work on PX4
Plane: use voltage_average() for RSSI

this fixes it on PX4
Plane: removed scaling of pitot source
Copter: fixed RSSI reading on PX4

needed for different analog input scaling
AP_HAL: removed scaling factor on analog sources

these are not use anymore, as voltage_average() is used instead