Add Structural similarity index measure image filter

[dzenanz]

SSIM is described on wikipedia: https://en.wikipedia.org/wiki/Structural_similarity_index_measure

We should implement it in ITK. The filter should be multi-dimensional, template-parametrized by the input images. Run-time parameters should be:

• Neighborhood used for computation
• relative weights of luminance, contrast, and structure
• array of scale weights (simplifying to ordinary SSIM when only one element is provided)