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 .