ekf2: set uncertainty of global origin to 0 when fusing GNSS data

When global measurements are fused in the filter, the local position
naturally contains the global position uncertainty.

src/modules/ekf2/EKF/gnss_height_control.cpp

@@ -142,6 +142,7 @@ void Ekf::controlGnssHeightFusion(const gpsSample &gps_sample)
 
 					_information_events.flags.reset_hgt_to_gps = true;
 					resetVerticalPositionTo(-measurement, measurement_var);
+					_gpos_origin_epv = 0.f; // The uncertainty of the global origin is now contained in the local position uncertainty
 					bias_est.reset();
 
 				} else {

src/modules/ekf2/EKF/gps_control.cpp

@@ -248,6 +248,7 @@ void Ekf::controlGpsFusion(const imuSample &imu_delayed)
 					// reset position
 					_information_events.flags.reset_pos_to_gps = true;
 					resetHorizontalPositionTo(position, pos_obs_var);
+					_gpos_origin_eph = 0.f; // The uncertainty of the global origin is now contained in the local position uncertainty
 					_aid_src_gnss_pos.time_last_fuse = _time_delayed_us;
 				}