BUG: Fix uninitialized variable in GradientRecursiveGaussianImageFilter

In GradientRecursiveGaussianImageFilter::TransformOutputPixel, the
correctedGradient variable is not fully initialized until the outer loop
completes.

Change-Id: Ia2a70086b83958320f7e159658ada7df98a753b2
DOC: HelloWorld Comments Edit for ITKSoftwareGuide

Corrected the use of an apostrophe and replaced 'Insight' with
ITK. It seems it should be either the whole thing 'Insight
Registration and Segmentation Toolkit' or ITK for consistency.

Change-Id: Ibac23d4b72290387d701d6707cb4015ba9901062
Merge topic 'Uninitialized-GradRecurGausFilter'

fbabe3ed BUG: Fix uninitialized variable in GradientRecursiveGaussianImageFilter
Merge topic 'HelloWorldCommentsEdit'

86552faa DOC: HelloWorld Comments Edit for ITKSoftwareGuide
DOC: Fix documentation in VoronoiDiagram2DGenerator

To be parsed, all Doxygen comments must be above method declarations,
not definitions.

Change-Id: I3037e24352f0d0d49b78b6cb0fb73f8cb26e086a
Merge topic 'Doc-VoronoiDiagram'

68ba6169 DOC: Fix documentation in VoronoiDiagram2DGenerator