EKF: publish the vertical position offset

EKF/estimator_interface.h

@@ -220,6 +220,9 @@ public:
 	// get GPS check status
 	virtual void get_gps_check_status(uint16_t *val) = 0;
 
+	// return the amount the local vertical position changed in the last height reset and the time of the reset
+	virtual void get_vert_pos_reset(float *delta, uint64_t *time_us) = 0;
+
 protected:
 
 	parameters _params;		// filter parameters