Test and clarify nn-unet baseline implementation

[jqmcginnis]

Scope

Ensure that the current implementation of the renaming scripts to generate the dataset for the nnUNet baseline in:
https://github.com/sct-pipeline/bavaria-quebec/tree/jqm/ms_lesion_nnunet/source/pipeline/baselines/nnunet
follows all current BIDS naming conventions and dataset modifications and is reproducible.

Procedure

• Test the renaming scripts with and without the cropped datasets, and with the optional sagittal image channel.
• Detail Readme with all steps required to do the training specifically for our dataset.
• Ask someone (i.e. the reviewer) to test the nnUNet naming scripts in the Pull Request to ensure the baseline generation is reproducible and could be done by someone at NeuroPoly as well (not only on TUM servers where e.g. the conda environment for nnUNet is already installed.)