get the non-zero weight observation names

        Returns
        -------
        nnz_obs_names : list
            a list of non-zero weighted observation names

def nnz_obs_names(self):
        """get the non-zero weight observation names

        Returns
        -------
        nnz_obs_names : list
            a list of non-zero weighted observation names

        """
        # nz_names = []
        # for w,n in zip(self.observation_data.weight,
        #                self.observation_data.obsnme):
        #     if w > 0.0:
        #         nz_names.append(n)
        obs = self.observation_data

        nz_names = list(obs.loc[obs.weight>0.0,"obsnme"])
        return nz_names