AnalogSource: return a float from read() method

this gives the full resolution when using the ADC backend. It also
adds scaling for the Arduino backend

libraries/AP_AnalogSource/AP_AnalogSource_ADC.cpp

@@ -1,9 +1,9 @@
 
 #include "AP_AnalogSource_ADC.h"
 
-int AP_AnalogSource_ADC::read(void)
+float AP_AnalogSource_ADC::read(void)
 {
-    int fullscale = _adc->Ch(_ch);
+    float fullscale = _adc->Ch(_ch);
-    int scaled = _prescale * fullscale;
+    float scaled = _prescale * fullscale;
     return scaled;
 }

libraries/AP_AnalogSource/AP_AnalogSource_ADC.h

@@ -10,7 +10,7 @@ class AP_AnalogSource_ADC : public AP_AnalogSource
     public:
     AP_AnalogSource_ADC( AP_ADC * adc, int ch, float prescale = 1.0 ) :
         _adc(adc), _ch(ch), _prescale(prescale) {}
-    int read(void);
+    float read(void);
 
     private:
     AP_ADC * _adc;

libraries/AP_AnalogSource/AP_AnalogSource_Arduino.cpp

@@ -1,8 +1,11 @@
+/// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
 
 #include "wiring.h"
 #include "AP_AnalogSource_Arduino.h"
 
-int AP_AnalogSource_Arduino::read(void)
+float AP_AnalogSource_Arduino::read(void)
 {
-    return analogRead(_pin);
+	float fullscale = analogRead(_pin);
+	float scaled = _prescale * fullscale;
+	return scaled;
 }

libraries/AP_AnalogSource/AP_AnalogSource_Arduino.h

@@ -1,3 +1,4 @@
+/// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
 
 #ifndef __AP_ANALOG_SOURCE_ARDUINO_H__
 #define __AP_ANALOG_SOURCE_ARDUINO_H__
@@ -7,11 +8,13 @@
 class AP_AnalogSource_Arduino : public AP_AnalogSource
 {
     public:
-    AP_AnalogSource_Arduino( int pin ) : _pin(pin) {}
+    AP_AnalogSource_Arduino( int pin, float prescale = 1.0 ) : 
-    int read(void);
+        _pin(pin), _prescale(prescale) {}
+    float read(void);
 
     private:
     int _pin;
+    float _prescale;
 };
 
 #endif // __AP_ANALOG_SOURCE_ARDUINO_H__

libraries/AP_AnalogSource/AnalogSource.h

@@ -5,7 +5,7 @@
 class AP_AnalogSource
 {
     public:
-    virtual int read(void) = 0;
+    virtual float read(void) = 0;
 };
 
 #endif // __ANALOG_SOURCE_H__