EKF: Add reporting of inertial dead-reckoning status

EKF/estimator_interface.h

@@ -175,6 +175,9 @@ public:
 	// return true if the global position estimate is valid
 	virtual bool global_position_is_valid() = 0;
 
+	// return true if the EKF is dead reckoning the position using inertial data only
+	virtual bool inertial_dead_reckoning() = 0;
+
 	// return true if the estimate is valid
 	// return the estimated terrain vertical position relative to the NED origin
 	virtual bool get_terrain_vert_pos(float *ret) = 0;