'The psi function for least trimmed mean
The analytic derivative of rho
Parameters
z : array-like
1d array
Returns
psi : array
psi(z) = z              for \|z\| <= c
psi(z) = 0              for \|z\| > c'

def psi(self, z):

    z = np.asarray(z)
    test = self._subset(z)
    return (test * z)
