code: private void writeObject(ObjectOutputStream out) throws IOException
      {
          out.writeInt(_length);
          out.write(_bytes, _offset, _length);
      }
docs: This is a CUSTOM serialisation method for the DataSlice
  object.
  
  We only need to write the bytes that are part of the payload.
  Any bytes that are not contained by the section defined by
  the offset and length values are not stored to disk. The
  length is also written for use upon reading the data from
  disk.
  
  @param out
  
  @exception IOException