'Inverse of the negative binomial transform
Parameters
z : array-like
The value of the inverse of the negative binomial link at `p`.
Returns
p : array
Mean parameters
Notes
g^(-1)(z) = exp(z)/(alpha*(1-exp(z)))'

def inverse(self, z):

    return ((-1) / (self.alpha * (1 - np.exp((- z)))))
