code: def nnz_obs_names(self):
          
          if self.__pst is not None:
              return self.pst.nnz_obs_names
          else:
              return self.jco.obs_names
docs: wrapper around pyemu.Pst.nnz_obs_names for listing non-zero
          observation names
  
          Returns
          -------
          nnz_obs_names : list
              pyemu.Pst.nnz_obs_names