function encodeLatLng(lat_lng) {
  return lat_lng.lat.toString() + "," + lat_lng.lng.toString();
}
Turn a LatLng object into a string representing its coordinates (to act as a graph node's ID).
@private

@param {LatLng} lat_lng - The coordinates to encode into a string.
@returns {string} - A string containing coordinates in the format of "Latitude,Longitude".