STK_RBF_SPHERICAL computes the spherical correlation function

 CALL: K = stk_rbf_spherical (H)

    computes the value of the spherical correlation function at distance H:

            /
            |  1 - 3/2 |h| + 1/2 |h|^3     if |h| < 1,
        K = |
            |  0                           otherwise.
            \

 CALL: K = stk_rbf_spherical (H, DIFF)

    computes the derivative of the spherical correlation function with
    respect the H if DIFF is equal to 1, and simply returns the value of the
    exponential correlation function if DIFF <= 0  (in which case it is
    equivalent to K = stk_rbf_spherical (H)).

 ADMISSIBILITY

    The spherical correlation is a valid correlation function in
    dimension d <= 3.