grib/src/main/java/ucar/nc2/grib/collection/Grib2PartitionBuilderFromIndex.java
@Nullable
  static PartitionCollectionMutable openMutablePCFromIndex(String name, RandomAccessFile raf,
      FeatureCollectionConfig config, org.slf4j.Logger logger) {

    Grib2PartitionBuilderFromIndex builder = new Grib2PartitionBuilderFromIndex(name, config, logger);
    if (builder.readIndex(raf))
      return builder.pc;

    return null;
  }
@Nullable
  static PartitionCollectionMutable openMutablePCFromIndex(String name, RandomAccessFile raf,
      FeatureCollectionConfig config, org.slf4j.Logger logger) {

    Grib2PartitionBuilderFromIndex builder = new Grib2PartitionBuilderFromIndex(name, config, logger);
    if (builder.readIndex(raf))
      return builder.pc;

    return null;
  }
read in the index, index raf already open; return null on failure