compute the difference between the full sample covariance and its separable approximation
difference_fullcov(Data)a (non-empty) N x d1 x d2 array of data values. The first
direction indices the \(N\) observations, each consisting of a d1 x d2
discretization of the surface, so that Data[i,,] corresponds to the
i-th observed surface.
A d1 x d2 x d1 x d2 array, where d1 = nrow(Data) and
d2 = ncol(Data).
This is an internal function.