'Gaussian deviance function
Parameters
endog : array-like
Endogenous response variable
mu : array-like
Fitted mean response variable
iweights : array-like
1d array of weights. The default is 1.
scale : float, optional
An optional scale argument. The default is 1.
Returns
deviance : float
The deviance function at (endog,mu,iweights,scale)
as defined below.
Notes
.. math::
D = \sum_i iweights_i * (Y_i - \mu_i)^2 / scale'

def deviance(self, endog, mu, iweights=1.0, scale=1.0):

    return (np.sum((iweights * ((endog - mu) ** 2))) / scale)
