StructureTensor speed up.

[phcerdan]

The computation of large StructureTensor is slow. For example when used with a large number of Riesz components in the related ImageToImageFilter. See #62 for the ongoing renaming and refactor.

• NeighborOperators are slow. Upload profiler result to show it.

• Check if we can speed it up avoiding boundary checking: https://itk.org/Doxygen/html/structitk_1_1NeighborhoodAlgorithm_1_1ImageBoundaryFacesCalculator.html

• Explore creating a compile type version, using FixedMatrix instead of VariableSizeMatrix. The number of inputs would be a template parameter.