Add covariance matrix in PV measurement
It would be nice to have the covariance matrix available in the PV
measurements.
Usually GNSS data comes with covariances or figure of merits of position
and velocity but they cannot be used yet in Orekit.
These covariances would integrate well in the noise matrix of a Kalman
filter.
One question though, should it one big 6x6 matrix or two smaller 3x3
matrices.
(from redmine: issue id 374, created on 2018-02-14, closed on 2018-06-04)