ers PARAM. The output matrix K has size NX x NY, where NX is the
   number of rows in X and NY the number of rows in Y. The vector of
   parameters must have DIM + 1 elements, where DIM is the common number
   of columns of X and Y:

    * PARAM(1) = log (SIGMA ^ 2), where SIGMA is the standard deviation,

    * PARAM(1+i) = - log (RHO(i)), where RHO(i) is the range parameter for
      the i-th dimension.

 CALL: dK = stk_materncov52_aniso (PARAM, X, Y, DIFF)

   computes the derivative of the covariance ma