code: def _set_meta(self):
          
          # we want to set meta-fields that are shared by ALL params in the PS
          for field in _meta_fields_twig:
              keys_for_this_field = set([getattr(p, field)
                                         for p in self.to_list()
                                         if getattr(p, field) is not None])
              if len(keys_for_this_field)==1:
                  setattr(self, '_'+field, list(keys_for_this_field)[0])
              else:
                  setattr(self, '_'+field, None)
docs: set the meta fields of the ParameterSet as those that are shared
          by ALL parameters in the ParameterSet.  For any fields that are
          not