code: def _kurtosis(self, x, z, d=0):
          
          val = self._poly(z, self._kurtosis_coeffs(d))
          return (np.tanh(val)+1)/12.*(3 - 6*x**2 + x**4)
docs: returns the kurtosis parameter for direction d, d=0 is rho, d=1 is z