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