STK_RBF_MATERN computes the Matern correlation function.

 CALL: K = stk_rbf_matern (NU, H)

    computes the value of the Matern correlation function of order NU at
    distance H. Note that the Matern correlation function is a valid
    correlation function for all dimensions.

 CALL: K = stk_rbf_matern (NU, H, DIFF)

    computes the derivative of the Matern correlation function of order NU, at
    distance H, with respect to the order NU if DIFF is equal to 1, or with
    respect the distance H if DIFF is equal to 2. (If DIFF is equal to -1,
    this is the same as K = stk_rbf_matern(NU, H).)

 See also: stk_rbf_matern32, stk_rbf_matern52