property decorated method to get a new ObservationEnsemble
        of only non-zero weighted observations

        Returns
        -------
        ObservationEnsemble : ObservationEnsemble

def nonzero(self):
        """ property decorated method to get a new ObservationEnsemble
        of only non-zero weighted observations

        Returns
        -------
        ObservationEnsemble : ObservationEnsemble

        """
        df = self.loc[:,self.pst.nnz_obs_names]
        return ObservationEnsemble.from_dataframe(df=df,
                        pst=self.pst.get(obs_names=self.pst.nnz_obs_names))