code: def nnz_obs_groups(self):
          
          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
docs: 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