STYLE: Improve readability and const correctness of ImageIORegion.

Change-Id: I9254bc96980505f4306608b7b45f9d021cb6e4bb
COMP: Ignore itkIndex.h -Warray-bounds warnings on GCC 4.9.

These appear to be false positives.  Example warning:

In file included from /var/lib/jenkins/jobs/ITK-v4-Testing-32-chroot/workspace/MyTests/ITK/Modules/Core/Common/include/itkContinuousIndex.h:22:0,
                 from /var/lib/jenkins/jobs/ITK-v4-Testing-32-chroot/workspace/MyTests/ITK/Modules/Core/Common/include/itkImageRegion.h:34,
                 from /var/lib/jenkins/jobs/ITK-v4-Testing-32-chroot/workspace/MyTests/ITK/Modules/Core/Common/include/itkImage.h:21,
                 from /var/lib/jenkins/jobs/ITK-v4-Testing-32-chroot/workspace/MyTests/ITK/Modules/Core/Common/include/itkImageSource.h:32,
                 from /var/lib/jenkins/jobs/ITK-v4-Testing-32-chroot/workspace/MyTests/ITK/Modules/Filtering/ImageSources/include/itkGenerateImageSource.h:22,
                 from /var/lib/jenkins/jobs/ITK-v4-Testing-32-chroot/workspace/MyTests/ITK/Modules/Filtering/ImageSources/include/itkGaborImageSource.h:21,
                 from /var/lib/jenkins/jobs/ITK-v4-Testing-32-chroot/workspace/MyTests/ITK/Modules/Filtering/ImageSources/test/itkGaborImageSourceTest.cxx:18:
/var/lib/jenkins/jobs/ITK-v4-Testing-32-chroot/workspace/MyTests/ITK/Modules/Core/Common/include/itkIndex.h: In member function 'void itk::ImageFileWriter<TInputImage>::GenerateData() [with TInputImage = itk::Image<float, 3u>]':
/var/lib/jenkins/jobs/ITK-v4-Testing-32-chroot/workspace/MyTests/ITK/Modules/Core/Common/include/itkIndex.h:226:19: warning: array subscript is above array bounds [-Warray-bounds]
   { return m_Index[dim]; }

Change-Id: Id7fe6616ba7b8938d1d9091b6515f4c77e787df5
Merge topic 'array-bounds'

66fc498f STYLE: Improve readability and const correctness of ImageIORegion.
Merge topic 'array-bounds'

4cd06116 COMP: Ignore itkIndex.h -Warray-bounds warnings on GCC 4.9.