BUG: Finalize transition to ITK MGH IO fixing mgh test. See 0003410

Fix failing test py_nomainwindow_test_slicer_mgh.

This commit fixes a regression introduced by r24539 (ENH: Convert to
using ITK version of MGHIO ) by explicitly adding MGHIO to list of IO
modules that should be loaded by ITKFactoryRegistration library.

Thanks: Bradley Lowekamp <blowekamp@mail.nih.gov>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24546 3bd1e089-480b-0410-8dfb-8563597acbee