Export trackline data to GeoJSON file

def export(cls, folder, particles, datetimes):
        """
            Export trackline data to GeoJSON file
        """
        normalized_locations = [particle.normalized_locations(datetimes) for particle in particles]

        track_coords = []
        for x in xrange(0, len(datetimes)):
            points = MultiPoint([loc[x].point.coords[0] for loc in normalized_locations])
            track_coords.append(points.centroid.coords[0])

        ls = LineString(track_coords)

        if not os.path.exists(folder):
            os.makedirs(folder)
        filepath = os.path.join(folder, "trackline.geojson")
        f = open(filepath, "wb")
        f.write(json.dumps(mapping(ls)))
        f.close()
        return filepath