EKF: add vertical position derivative output

Add calculation of a vertical position derivative to the output predictor. This will have degraded tracking relative to the EKF states, but the velocity will be closer to the first derivative of the position and reduce the effect inertial prediction errors on control loops that are operating in a pure velocity feedback mode.
Move calculation of IMU offset angular rate correction out of velocity accessor and into output predictor.
Provide separate accessor for vertical position derivative.

EKF/RingBuffer.h

@@ -113,6 +113,11 @@ public:
 		return _buffer[_tail];
 	}
 
+	unsigned get_oldest_index()
+	{
+		return _tail;
+	}
+
 	inline data_type get_newest()
 	{
 		return _buffer[_head];