code: def loads_sector(self, sector='res'):
          
          
          for load in self._loads:
              if (sector == 'res') and (load.string_id is not None):
                  yield load
              elif (sector == 'ria') and (load.string_id is None):
                  yield load
docs: Returns a generator for iterating over grid's sectoral loads
          
          Parameters
          ----------
          sector: String
              possible values::
                  
                  'res' (residential),
                  'ria' (retail, industrial, agricultural)
  
          Yields
          -------
          int 
              Generator for iterating over loads of the type specified in `sector`.