'Ramsay\'s Ea weighting function for the IRLS algorithm
The psi function scaled by z
Parameters
z : array-like
1d array
Returns
weights : array
weights(z) = exp(-a*\|z\|)'

def weights(self, z):

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