AP_HAL: added set_stop_pin() and set_settle_time() interfaces to analogin

this is for multiple sonars on one vehicle, to prevent them
interfering with each other

libraries/AP_HAL/AnalogIn.h

@@ -10,6 +10,20 @@ public:
     virtual float read_latest() = 0;
     virtual void set_pin(uint8_t p) = 0;
 
+    // optionally allow setting of a pin that stops the device from
+    // reading. This is needed for sonar devices where you have more
+    // than one sonar, and you want to stop them interfering with each
+    // other. It assumes that if held low the device is stopped, if
+    // held high the device starts reading.    
+    virtual void set_stop_pin(uint8_t p) = 0;
+
+    // optionally allow a settle period in milliseconds. This is only
+    // used if a stop pin is set. If the settle period is non-zero
+    // then the analog input code will wait to get a reading for that
+    // number of milliseconds. Note that this will slow down the
+    // reading of analog inputs.
+    virtual void set_settle_time(uint16_t settle_time_ms) = 0;
+
     // return a voltage from 0.0 to 5.0V, scaled
     // against a reference voltage
     virtual float voltage_average() = 0;