AP_AHRS: added get_projected_position()

this is used to project the position forward by the GPS lag

libraries/AP_AHRS/AP_AHRS.cpp

@@ -224,3 +224,15 @@ Vector2f AP_AHRS::groundspeed_vector(void)
     }
     return Vector2f(0.0f, 0.0f);
 }
+
+/*
+  get position projected by groundspeed and heading
+ */
+bool AP_AHRS::get_projected_position(struct Location *loc)
+{
+        if (!get_position(loc)) {
+		return false;
+        }
+        location_update(loc, degrees(yaw), _gps->ground_speed * 0.01 * _gps->get_lag());
+        return true;
+}

libraries/AP_AHRS/AP_AHRS.h

@@ -127,6 +127,10 @@ public:
         return true;
     }
 
+    // get our projected position, based on our GPS position plus
+    // heading and ground speed
+    bool get_projected_position(struct Location *loc);
+
     // return a wind estimation vector, in m/s
     virtual Vector3f wind_estimate(void) {
         return Vector3f(0,0,0);