AP_AnalogSource: make read_average() return a float

this allows it to take advantage of better than 1 bit resolution by
averaging

libraries/AP_AnalogSource/AP_AnalogSource_Arduino.cpp

@@ -104,7 +104,7 @@ uint16_t AP_AnalogSource_Arduino::read_vcc(void)
 // time read_average() was called. This gives a very cheap
 // filtered value, as new values are produced at 500/N Hz
 // where N is the total number of analog sources
-uint16_t AP_AnalogSource_Arduino::read_average(void)
+float AP_AnalogSource_Arduino::read_average(void)
 {
     uint16_t sum;
     uint8_t sum_count;
@@ -119,7 +119,7 @@ uint16_t AP_AnalogSource_Arduino::read_average(void)
     pins[_pin_index].sum = 0;
     pins[_pin_index].sum_count = 0;
     sei();
-    return sum / sum_count;
+    return sum / (float)sum_count;
 }
 
 // read with the prescaler. This uses the averaged value since

libraries/AP_AnalogSource/AP_AnalogSource_Arduino.h

@@ -32,7 +32,7 @@ class AP_AnalogSource_Arduino : public AP_AnalogSource
 
     // read the average 16 bit ADC value since
     // we last called read_average().
-    uint16_t read_average(void);
+    float read_average(void);
 
     private:
     uint8_t _pin_index;