AP_AHRS: make optflow available via AHRS

same pattern as compass and airspeed sensor

libraries/AP_AHRS/AP_AHRS.h

@@ -26,6 +26,7 @@
 #include <inttypes.h>
 #include <AP_Compass.h>
 #include <AP_Airspeed.h>
+#include <AP_OpticalFlow.h>
 #include <AP_GPS.h>
 #include <AP_InertialSensor.h>
 #include <AP_Baro.h>
@@ -63,6 +64,7 @@ public:
         yaw_sensor(0),
         _vehicle_class(AHRS_VEHICLE_UNKNOWN),
         _compass(NULL),
+        _optflow(NULL),
         _airspeed(NULL),
         _compass_last_update(0),
         _ins(ins),
@@ -136,6 +138,14 @@ public:
     const Compass* get_compass() const {
         return _compass;
     }
+
+    void set_optflow(const OpticalFlow *optflow) {
+        _optflow = optflow;
+    }
+
+    const OpticalFlow* get_optflow() const {
+        return _optflow;
+    }
         
     // allow for runtime change of orientation
     // this makes initial config easier
@@ -393,6 +403,9 @@ protected:
     // pointer to compass object, if available
     Compass         * _compass;
 
+    // pointer to OpticalFlow object, if available
+    const OpticalFlow *_optflow;
+
     // pointer to airspeed object, if available
     AP_Airspeed     * _airspeed;