typo in MapInfoDataset

[zkytony]

Noting a minor issue. In a script in the dataset SL_OSM_Dataset/mapinfo/map_info_dataset.py, the class MapInfoDataset has a visualize function in which

        center = mapinfo.center_of_mass(landmark_symbol, map_name)

should be

        center = self.center_of_mass(landmark_symbol, map_name)

This function wasn't used anywhere else than in the map_info_dataset.py itself for testing (and because mapinfo object was created in __main__, it didn't throw an exception when running map_info_dataset.py).