get the zero-weighted observation names

        Returns
        -------
         zero_weight_obs_names : list
             a list of zero-weighted observation names

def zero_weight_obs_names(self):
        """ get the zero-weighted observation names

        Returns
        -------
         zero_weight_obs_names : list
             a list of zero-weighted observation names

        """
        self.observation_data.index = self.observation_data.obsnme
        groups = self.observation_data.groupby(
                self.observation_data.weight.apply(lambda x: x==0.0)).groups
        if True in groups:
            return list(self.observation_data.loc[groups[True],"obsnme"])
        else:
            return []