code: public static <A, B, C> FeatureGenerator<A, C> convertingFeatureGenerator(
        FeatureGenerator<B, C> generator, Function<A, B> converter) {
      return new ConvertingFeatureGenerator<A, B, C>(generator, converter);
    }
docs: Gets a feature generator that first converts the data,
  then applies a given feature generator.
  
  @param generator
  @param converter
  @return