- Uses a refPtr to reference external content.
  Useful (for example) when writing data without copying.
  Reading into external locations is not implemented
  (no current requirement for that).

    * IOFieldRef -> IOField
    * IOListRef -> IOList
    * IOmapDistributePolyMeshRef -> IOmapDistributePolyMesh

  Eg,

    labelList addressing = ...;

    io.rename("cellProcAddressing");
    IOListRef<label>(io, addressing).write();

  Or,
    primitivePatch patch = ...;
    IOFieldRef<vector>(io, patch.localPoints()).write();