Adding voxel-space sigma specification to itk::SmoothingRecursiveYvvGaussianImageFilter

Rather than diverting NaN for DiscreteGaussian thread any more, I’d like to bring up a feature I’m adding to the RecursiveYvv filter.

I’ve got a draft available here

Right now I’m having a bit of C++ problem getting the setting to pass all the way into the right place, I’d appreciate some help here.

Goal of this is to make SmoothingRecursiveYvvGaussianImageFilter a drop-in replacement for DiscreteGaussian