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/common.h

@@ -79,9 +79,16 @@ struct ext_vision_message {
 
 struct outputSample {
 	Quaternion  quat_nominal;	// nominal quaternion describing vehicle attitude
-	Vector3f    vel;	// NED velocity estimate in earth frame in m/s
-	Vector3f    pos;	// NED position estimate in earth frame in m/s
-	uint64_t 	time_us;	// timestamp in microseconds
+	Vector3f    vel;		// NED velocity estimate in earth frame in m/s
+	Vector3f    pos;		// NED position estimate in earth frame in m/s
+	uint64_t    time_us;		// timestamp in microseconds
+};
+
+struct outputVert {
+	float	    vel_d;		// D velocity calculated using alternative algorithm
+	float	    vel_d_integ;	// Integral of vel_d
+	float	    dt;			// delta time in seconds
+	uint64_t    time_us;		// timestamp in microseconds
 };
 
 struct imuSample {