Use the Radon transform to measure spinal cord morphometry

[valosekj]

Use the Radon transform to measure spinal cord morphometry.

Brief context

Currently, sct_process_segmentation uses skimage.regionprops's elliptical approximation to measure AP and RL diameters. This approach has limitations in pathologies, e.g., when the spinal cord is compressed and rotated -- for example, the ellipse might not capture the shape of the compressed cord.
The idea is to apply the Radon transform (used in CT imaging) slice-wise along the SI direction on the spinal cord segmentation to get the angle we could rotate the spinal cord segmentation by.

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Some relevant issues and discussions

spinalcordtoolbox/spinalcordtoolbox#4939
spinalcordtoolbox/spinalcordtoolbox#4958
spinalcordtoolbox/spinalcordtoolbox#4943
spinalcordtoolbox/spinalcordtoolbox#4282