Optional: saggital channel registration for nn-unet (and other models) using sct_register_multimodal

[jqmcginnis]

Hi all,

as shortly discussed during our last meeting, I would like to use the sagittally acquired T2w as a second input channel for the nn-unet (in a second/additional configuration). NN-Unet requires that both input images (axial and sagittal) are registered, so I would include this in the pre-processing script, by using sct_register_multimodal.

Any thoughts on this? If you agree, I am happy to implement this in a PR.