public static FeatureGroup newDiskBackedFeatureGroup(TileOrder tileOrder, Path tempDir, Profile profile,
    PlanetilerConfig config, Stats stats) {
    return new FeatureGroup(
      new ExternalMergeSort(tempDir, config, stats),
      tileOrder, profile, config, stats
    );
  }
Returns a feature grouper that writes all elements to disk in chunks, sorts each chunk, then reads back in order
from those chunks. Suitable for making maps up to planet-scale.
planetiler-core/src/main/java/com/onthegomap/planetiler/collection/FeatureGroup.java