Fixing pattern covariance term in initiating state of `SiTrajectoryElement_xk` (fixes ATLSWUPGR-185)

Fixes ATLSWUPGR-185.

The un-initialised term in the covariance matrix brings to undefined behaviour in Runge-Kutta progatation of parameters. This MR fixes it.

@jmasik @bwynne @ewatton @pagessin @tstreble @demelian