vtkPeriodicDataArray can now return an iterator.
vtkPeriodicDataArray can now return an iterator.

This fix is in reference to bug report 0015518. This fix enables
vtkPeriodicDataArrays to be written to XML, albeit inefficiently.

sankhesh/VTK,keithroe/vtkoptix,sumedhasingla/VTK,SimVascular/VTK,sankhesh/VTK,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,gram526/VTK,mspark93/VTK,sumedhasingla/VTK,mspark93/VTK,gram526/VTK,mspark93/VTK,sumedhasingla/VTK,sankhesh/VTK,mspark93/VTK,sumedhasingla/VTK,gram526/VTK,sankhesh/VTK,SimVascular/VTK,gram526/VTK,gram526/VTK,sumedhasingla/VTK,mspark93/VTK,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,sumedhasingla/VTK,sankhesh/VTK,mspark93/VTK,sumedhasingla/VTK,sumedhasingla/VTK,SimVascular/VTK,mspark93/VTK,mspark93/VTK,SimVascular/VTK,gram526/VTK,gram526/VTK,keithroe/vtkoptix,sankhesh/VTK,sankhesh/VTK,SimVascular/VTK,sankhesh/VTK,gram526/VTK,keithroe/vtkoptix