'Tukey\'s biweight weighting function for the IRLS algorithm
The psi function scaled by z
Parameters
z : array-like
1d array
Returns
weights : array
psi(z) = (1 - (z/c)**2)**2          for \|z\| <= R
psi(z) = 0                          for \|z\| > R'

def weights(self, z):

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