Update the posterior and estimates after a label is sampled

        Parameters
        ----------
        ell : int
            sampled label: 0 or 1

        k : int
            index of stratum where label was sampled

def update(self, ell, k):
        """Update the posterior and estimates after a label is sampled

        Parameters
        ----------
        ell : int
            sampled label: 0 or 1

        k : int
            index of stratum where label was sampled
        """
        self.alpha_[k] += ell
        self.beta_[k] += 1 - ell

        self._calc_theta()
        if self.store_variance:
            self._calc_var_theta()