* EKF: Improve covariance prediction stability

Eliminates collapse of vertical velocity state variance due to rounding errors that can occur under some operating conditions.

* EKF: Fix typo

* test: Fix initialisation test cases

Provide sufficient time for variances to stabilise and fix calculation of reference quaternion for alignment.

* test: Allow for accumulated rounding error in IMU sampling test

* test: Allow sufficient time for quaternion variances to reduce after initial alignment

* test: Increase allowance for tilt alignment delay and inertial nav errors

* test: Increase allowance for tilt alignment delay and inertial nav errors

* adpat reset velocity test

* test: update change indication file

* test: Adjust tests to handle alignment time and prediction errors

* README.md: Add documentation for change indicator test