code: def add_mv_grid_district(self, mv_grid_district):
          
          # TODO: use setter method here (make attribute '_mv_grid_districts' private)
          if mv_grid_district not in self.mv_grid_districts():
              self._mv_grid_districts.append(mv_grid_district)
docs: Adds a MV grid_district to _mv_grid_districts if not already existing