STK_DISCRETECOV computes a covariance matrix for a discrete model

 CALL: K = stk_discretecov (PARAM, X, Y)

    computes the covariance matrix K between the sets of locations X and
    Y, using the discrete model (a kriging model on a discrete space)
    parameters PARAM. The inputs arguments X and Y are expected to be
    vectors of indices. The output matrix K has size NX x NY, where NX is
    the length of X and NY the length of Y. PARAM must contain a field
    PARAM.K which is the full covariance matrix on the discrete input
    space.

 CALL: K = stk_discretecov (PARAM, X, Y, DIFF, PAIRWISE)

    computes the covariance *vector* between the sets of locations X and
    Y if PAIRWISE is TRUE, and the covariance *matrix* otherwise. In the
    first case, the output K is a vector of length N, where N is the
    common number of rows of X and Y. In the second, it is an NX x NY
    matrix as above. The argument DIFF has no effect, but must be equal
    to -1 since this is not a parametric model.