COMP: Improve const correctness of GradientRecursiveGaussianImageFilter.

Change-Id: I294fb347d65e584475567c1bf3e8152b32250cea
Merge topic 'Coverity_itkGradientRecursiveGaussianImageFilter'

00c1a5d3 COMP: Improve const correctness of GradientRecursiveGaussianImageFilter.