Don't require `shape_dist_traveled` when loading GTFS data

[dan-mccabe]

read_in_gtfs() currently uses:

  req_cols = {
      "stop_times": [
          "arrival_time",
          "departure_time",
          "shape_dist_traveled",
          "stop_id",
      ],
      "shapes": ["shape_dist_traveled"],
  }
  feed = Feed.from_dir(path_to_feed, columns=req_cols)

However, we should have no problem analyzing feeds that lack shape_dist_traveled since we can estimate it ourselves. I'll remove this requirement and test that everything runs successfully when this column is missing.