AP_GPS: added velocity_down() call

on GPSes with raw velocity support this will be used for DCM
correction

libraries/AP_GPS/GPS.cpp

@@ -59,6 +59,7 @@ GPS::update(void)
                 // the GPS is able to give us velocity numbers directly
                 _velocity_north = _vel_north * 0.01;
                 _velocity_east  = _vel_east * 0.01;
+                _velocity_down  = _vel_down * 0.01;
             } else {
                 float gps_heading = ToRad(ground_course * 0.01);
                 float gps_speed   = ground_speed * 0.01;
@@ -69,6 +70,9 @@ GPS::update(void)
 
                 _velocity_north = gps_speed * cos_heading;
                 _velocity_east  = gps_speed * sin_heading;
+
+				// no good way to get descent rate
+				_velocity_down  = 0;
             }
         }
     }

libraries/AP_GPS/GPS.h

@@ -128,6 +128,9 @@ public:
     float velocity_east(void)  {
         return _status == GPS_OK ? _velocity_east  : 0;
     }
+    float velocity_down(void)  {
+        return _status == GPS_OK ? _velocity_down  : 0;
+    }
 
     // last ground speed in m/s. This can be used when we have no GPS
     // lock to return the last ground speed we had with lock
@@ -141,6 +144,9 @@ public:
     // the time we got our last fix in system milliseconds
     uint32_t last_fix_time;
 
+	// return true if the GPS supports raw velocity values
+
+
 protected:
     Stream      *_port;                 ///< port the GPS is attached to
 
@@ -222,6 +228,7 @@ private:
     // components of the velocity, in m/s
     float _velocity_north;
     float _velocity_east;
+    float _velocity_down;
 };
 
 inline int32_t