Log PDF for Inverse Gamma prior

        Parameters
        ----------
        x : float
            Latent variable for which the prior is being formed over

        Returns
        ----------
        - log(p(x))

def logpdf(self, x):
        """
        Log PDF for Inverse Gamma prior

        Parameters
        ----------
        x : float
            Latent variable for which the prior is being formed over

        Returns
        ----------
        - log(p(x))
        """
        if self.transform is not None:
            x = self.transform(x)       
        return (-self.alpha-1)*np.log(x) - (self.beta/float(x))