@Override
  public long diskUsageBytes() {
    return sorter.diskUsageBytes();
  }
Reads temp features using {@code threads} parallel threads and merges into a sorted list.

@param threads The number of parallel read threads to spawn
@return a {@link Reader} with a handle to the new read threads that were spawned, and in {@link Iterable} that can
        be used to iterate over the results.
planetiler-core/src/main/java/com/onthegomap/planetiler/collection/FeatureGroup.java