AP_AHRS: added groundspeed() method

this gives ground speed in m/s from GPS

libraries/AP_AHRS/AP_AHRS.h

@@ -191,6 +191,14 @@ public:
     // return a ground vector estimate in meters/second, in North/East order
     Vector2f groundspeed_vector(void);
 
+    // return ground speed estimate in meters/second. Used by ground vehicles.
+    float groundspeed(void) const {
+        if (!_gps || _gps->status() <= GPS::NO_FIX) {
+            return 0.0f;
+        }
+        return _gps->ground_speed_cm * 0.01f;
+    }
+
     // return true if we will use compass for yaw
     virtual bool use_compass(void) const { return _compass && _compass->use_for_yaw(); }