private static <I, O> Converter<I, O> converter(Class<I> in, Class<O> out, Function<I, O> fn) {
    return new Converter<>(in, out, fn);
  }
Utility for convert between types in a forgiving way (parse strings to get a number, call toString to get a string,
etc.).
planetiler-custommap/src/main/java/com/onthegomap/planetiler/custommap/TypeConversion.java