get the observation groups that contain at least one non-zero weighted
         observation

        Returns
        -------
        nnz_obs_groups : list
            a list of observation groups that contain at
            least one non-zero weighted observation

def nnz_obs_groups(self):
        """ get the observation groups that contain at least one non-zero weighted
         observation

        Returns
        -------
        nnz_obs_groups : list
            a list of observation groups that contain at
            least one non-zero weighted observation

        """
        og = []
        obs = self.observation_data
        for g in self.obs_groups:
            if obs.loc[obs.obgnme==g,"weight"].sum() > 0.0:
                og.append(g)
        return og