code: def to_geojson(self, filename, proj, metadata=None):
          
          if metadata is None:
              metadata = {}
          json_obj = {"type": "FeatureCollection", "features": [], "properties": {}}
          json_obj['properties']['times'] = self.times.tolist()
          json_obj['properties']['dx'] = self.dx
          json_obj['properties']['step'] = self.step
          json_obj['properties']['u'] = self.u.tolist()
          json_obj['properties']['v'] = self.v.tolist()
          for k, v in metadata.items():
              json_obj['properties'][k] = v
          for t, time in enumerate(self.times):
              feature = {"type": "Feature",
                         "geometry": {"type": "Polygon"},
                         "properties": {}}
              boundary_coords = self.boundary_polygon(time)
              lonlat = np.vstack(proj(boundary_coords[0], boundary_coords[1], inverse=True))
              lonlat_list = lonlat.T.tolist()
              if len(lonlat_list) > 0:
                  lonlat_list.append(lonlat_list[0])
              feature["geometry"]["coordinates"] = [lonlat_list]
              for attr in ["timesteps", "masks", "x", "y", "i", "j"]:
                  feature["properties"][attr] = getattr(self, attr)[t].tolist()
              feature["properties"]["attributes"] = {}
              for attr_name, steps in self.attributes.items():
                  feature["properties"]["attributes"][attr_name] = steps[t].tolist()
              json_obj['features'].append(feature)
          file_obj = open(filename, "w")
          json.dump(json_obj, file_obj, indent=1, sort_keys=True)
          file_obj.close()
          return
docs: Output the data in the STObject to a geoJSON file.
  
          Args:
              filename: Name of the file
              proj: PyProj object for converting the x and y coordinates back to latitude and longitue values.
              metadata: Metadata describing the object to be included in the top-level properties.