'Least trimmed mean weighting function for the IRLS algorithm
The psi function scaled by z
Parameters
z : array-like
1d array
Returns
weights : array
weights(z) = 1             for \|z\| <= c
weights(z) = 0             for \|z\| > c'

def weights(self, z):

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