ekf2: Use ecl library with recent bug fixes and enhancements.

Bug Fixes:

https://github.com/PX4/ecl/pull/586 - EKF: fix covariance and output filter buffer initialization
https://github.com/PX4/ecl/pull/590 - EKF: Fix innovation in fuseDeclination()

Enhancements:

https://github.com/PX4/ecl/pull/543 - ekf_helper: add more useful methods to interface with the covariances
https://github.com/PX4/ecl/pull/588 - Add unit tests for DataValidator

Note:

https://github.com/PX4/ecl/pull/543  has required a change to how the state variances are accessed .

src/lib/ecl

@@ -1 +1 @@
-Subproject commit a85d3a43edc4ac4b85ae21159510b9c26f7359e6
+Subproject commit a892ececf8490b21aa8917bc243b2bc441af6a87

src/modules/ekf2/ekf2_main.cpp

@@ -1423,7 +1423,7 @@ void Ekf2::run()
 
 				// Get covariances to vehicle odometry
 				float covariances[24];
-				_ekf.get_covariances(covariances);
+				_ekf.covariances_diagonal().copyTo(covariances);
 
 				// get the covariance matrix size
 				const size_t POS_URT_SIZE = sizeof(odom.pose_covariance) / sizeof(odom.pose_covariance[0]);
@@ -1540,7 +1540,7 @@ void Ekf2::run()
 			status.timestamp = now;
 			_ekf.get_state_delayed(status.states);
 			status.n_states = 24;
-			_ekf.get_covariances(status.covariances);
+			_ekf.covariances_diagonal().copyTo(status.covariances);
 			_ekf.get_gps_check_status(&status.gps_check_fail_flags);
 			// only report enabled GPS check failures (the param indexes are shifted by 1 bit, because they don't include
 			// the GPS Fix bit, which is always checked)