'The psi function for Huber\'s t estimator
The analytic derivative of rho
Parameters
z : array-like
1d array
Returns
psi : array
psi(z) = z      for \|z\| <= t
psi(z) = sign(z)*t for \|z\| > t'

def psi(self, z):

    z = np.asarray(z)
    test = self._subset(z)
    return ((test * z) + (((1 - test) * self.t) * np.sign(z)))
