'Derivative of the inverse link function g^(-1)(z).
Notes
This reference implementation gives the correct result but is
inefficient, so it can be overriden in subclasses.
Parameters
z : array-like
`z` is usually the linear predictor for a GLM or GEE model.
Returns
g\'^(-1)(z) : array
The value of the derivative of the inverse of the link function'

def inverse_deriv(self, z):

    return (1 / self.deriv(self.inverse(z)))
