STK_MAKE_MATCOV computes a covariance matrix (and a design matrix)

 CALL: [K, P] = stk_make_matcov (MODEL, X0)

    computes the covariance matrix K and the design matrix P for the model
    MODEL at the set of points X0, which is expected to be an N x DIM
    array. As a result, a matrix K of size N x N and a matrix P of size
    N x L are obtained, where L is the number of regression functions in
    the linear part of the model.

 CALL: K = stk_make_matcov (MODEL, X0, X1)

    computes the covariance matrix K for the model MODEL between the sets
    of points X0 and X1. The resulting K matrix is of size N0 x N1, where
    N0 is the number of rows of XO and N1 the number of rows of X1.

 BE CAREFUL:

    stk_make_matcov (MODEL, X0) and stk_makematcov (MODEL, X0, X0) are NOT
    equivalent if model.lognoisevariance > - inf  (in the first case, the
    noise variance is added on the diagonal of the covariance matrix).