code: Object unwrapListPrimitive(Object o)
    {
      assert isWrappedListPrimitive(o);
      Group g = (Group) o;
      return convertPrimitiveField(g, 0, binaryAsString);
    }
docs: Properly formed parquet lists when passed through {@link ParquetGroupConverter#convertField(Group, String)} can
  return lists which contain 'wrapped' primitives, that are a {@link Group} with a single, primitive field (see
  {@link ParquetGroupConverter#isWrappedListPrimitive(Object)})