'Weights for IRLS steps
Parameters
mu : array-like
The transformed mean response variable in the exponential family
Returns
w : array
The weights for the IRLS steps
Notes
.. math::
w = 1 / (g\'(\mu)^2  * Var(\mu))'

def weights(self, mu):

    return (1.0 / ((self.link.deriv(mu) ** 2) * self.variance(mu)))
