'The psi function for Tukey\'s biweight estimator
The analytic derivative of rho
Parameters
z : array-like
1d array
Returns
psi : array
psi(z) = z*(1 - (z/c)**2)**2        for \|z\| <= R
psi(z) = 0                           for \|z\| > R'

def psi(self, z):

    z = np.asarray(z)
    subset = self._subset(z)
    return ((z * ((1 - ((z / self.c) ** 2)) ** 2)) * subset)
