'Inverse of the power transform link function
Parameters
`z` : array-like
Value of the transformed mean parameters at `p`
Returns
`p` : array
Mean parameters
Notes
g^(-1)(z`) = `z`**(1/`power`)'

def inverse(self, z):

    if (self.power == 1):
        return z
    else:
        return np.power(z, (1.0 / self.power))
