also interpolate covariance matrix in LB
find the closest track point for input given in lb coordinates, somewhat slow when using velocities
work on calculating the Gaussian approximation in LB coordinates
apply LB covariance scale factors when reconstructing the variance
rename the LB covariance matrix to make it clear that it is rescaled; fix covariance multiplication
rename gaussApproxXY to gaussApprox, as it now also works for LB coordinates
marginalze the DF over missing directions in LB