AP_AHRS: added get_position_lag() call

provide position lag to libraries

libraries/AP_AHRS/AP_AHRS.cpp

@@ -211,3 +211,11 @@ bool AP_AHRS::get_projected_position(struct Location &loc)
         location_update(loc, degrees(yaw), _gps->ground_speed_cm * 0.01 * _gps->get_lag());
         return true;
 }
+
+/*
+  get the GPS lag in seconds
+ */
+float AP_AHRS::get_position_lag(void) const
+{
+    return _gps->get_lag();
+}

libraries/AP_AHRS/AP_AHRS.h

@@ -141,6 +141,9 @@ public:
     // heading and ground speed
     bool get_projected_position(struct Location &loc);
 
+    // return the estimated lag in our position due to GPS lag
+    float get_position_lag(void) const;
+
     // return a wind estimation vector, in m/s
     virtual Vector3f wind_estimate(void) {
         return Vector3f(0,0,0);