EKF: fix bug in sideslip fusion and reduce RAM usage
EKF: fix bug in sideslip fusion and reduce RAM usage

The bug meant the  Y velocity (state index 5)  covariance was not being updated correctly when sideslip was being used to constrain velocity drift (extended GPS loss).
The rework of the covariance update to reduce RAM follows the same pattern as adopted for other fusion processes

PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl