DEV: updated WM atlas generation to use the new sct_antsApplyTransforms
DEV: updated WM atlas generation to use the new sct_antsApplyTransforms

Former-commit-id: 0841d1301d38a321159588c826cbe575b4fc9329
Former-commit-id: a7ceeb2ebb80b137f82dbc0b9c86801a4d9ae677
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox