GradientDifference and NormalizedGradientCorrelation metrics use unimplemented 4D SobelOperator

[N-Dekker]

"GradientDifference" and "NormalizedGradientCorrelation" metrics both use SobelOperator:

elastix/Components/Metrics/GradientDifference/itkGradientDifferenceImageToImageMetric2.h

Line 209 in 2fb09c3

SobelOperator<FixedGradientPixelType, Self::FixedImageDimension> m_FixedSobelOperators[FixedImageDimension]{};

elastix/Components/Metrics/NormalizedGradientCorrelation/itkNormalizedGradientCorrelationImageToImageMetric.h

Line 192 in 2fb09c3

SobelOperator<FixedGradientPixelType, Self::FixedImageDimension> m_FixedSobelOperators[FixedImageDimension]{};

In general, ImageDimension may be 4, and elastix does support 4-D. However, when using ITK 5 (including ITK 5.4.4), itk::SobelOperator throws an exception for 4-D images, saying:

    itkExceptionMacro("The ND version of the Sobel operator is not yet implemented.  Currently only the 2D and 3D "
                      "versions are available.");

https://github.com/InsightSoftwareConsortium/ITK/blob/f98d5fac5e1d5ef694f3010f12bbbc2c792994c6/Modules/Core/Common/include/itkSobelOperator.hxx#L73-L74

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Thanks to Bradley Lowekamp (@blowekamp) for remarking that elastix does use itk::SobelOperator for 4-D! At InsightSoftwareConsortium/ITK#5718 (comment)