docs: 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