'The psi function for Ramsay\'s Ea estimator
The analytic derivative of rho
Parameters
z : array-like
1d array
Returns
psi : array
psi(z) = z*exp(-a*\|z\|)'

def psi(self, z):

    z = np.asarray(z)
    return (z * np.exp(((- self.a) * np.fabs(z))))
