Calculate the mean of a quantity within strata

        Parameters
        ----------
        values : array-like, shape=(n_items,n_class)
            array containing the values of the quantity for each item in the
            pool

        Returns
        -------
        numpy.ndarray, shape=(n_strata,n_class)
            array containing the mean value of the quantity within each stratum

def intra_mean(self, values):
        """Calculate the mean of a quantity within strata

        Parameters
        ----------
        values : array-like, shape=(n_items,n_class)
            array containing the values of the quantity for each item in the
            pool

        Returns
        -------
        numpy.ndarray, shape=(n_strata,n_class)
            array containing the mean value of the quantity within each stratum
        """
        # TODO Check that quantity is valid
        if values.ndim > 1:
            return np.array([np.mean(values[x,:], axis=0) for x in self.allocations_])
        else:
            return np.array([np.mean(values[x]) for x in self.allocations_])