difference_fullcov: compute the difference between the full sample covariance and its separable
compute the difference between the full sample covariance and its separable
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.
d1 x d2 x d1 x d2 array, where
d1 = nrow(Data) and
d2 = ncol(Data).
This is an internal function.