EKF: delay commencement of 3D mag fusion until clear of ground

Wait until enough height has been gained to be clear of ground based magnetic anomalies. Failure to do so can result in incorrect earth field initialisation.

EKF/ekf.cpp

@@ -98,6 +98,8 @@ Ekf::Ekf():
 	_ev_counter(0),
 	_time_last_mag(0),
 	_hgt_sensor_offset(0.0f),
+	_baro_hgt_offset(0.0f),
+	_last_on_ground_posD(0.0f),
 	_terrain_vpos(0.0f),
 	_terrain_var(1.e4f),
 	_hagl_innov(0.0f),
@@ -106,7 +108,6 @@ Ekf::Ekf():
 	_baro_hgt_faulty(false),
 	_gps_hgt_faulty(false),
 	_rng_hgt_faulty(false),
-	_baro_hgt_offset(0.0f),
 	_time_bad_vert_accel(0)
 {
 	_state = {};