'The robust criterion function for least trimmed mean.
Parameters
z : array-like
1d array
Returns
rho : array
rho(z) = (1/2.)*z**2    for \|z\| <= c
rho(z) = 0              for \|z\| > c'

def rho(self, z):

    z = np.asarray(z)
    test = self._subset(z)
    return ((test * (z ** 2)) * 0.5)
