Issue title: [sct_register_multimodal] slicereg fails.. but slicereg2d_translation works on my data (4slices)
 Issue body: data are in:
`````
/Volumes/folder_shared/BUGS/sct_register_multimodal/slicereg
`````
 Comments: 
 Comment 0: What is the command?
 Comment 1: simplest:
``sct_register_multimodal -i AxCaliber_1_1.nii.gz -d AxCaliber_2_1.nii.gz -p step=1,algo=slicereg2d_translation`

and 

``````
sct_register_multimodal -i AxCaliber_1_1.nii.gz -d AxCaliber_2_1.nii.gz -p step=1,algo=slicereg````
``````

{'source': 'GithubIssues', 'repo_name_with_owner': 'neuropoly/spinalcordtoolbox', 'issue_id': 94980189, 'repo_stars': 134.0}