OPT: sct_warp_template now using linear instead of spline for WM, GM and atlas
OPT: sct_warp_template now using linear instead of spline for WM, GM and atlas

Former-commit-id: 8ae3205a68211dbaf2f5816fc161fff32b8cd7cc
Former-commit-id: 334189928327cb9837249f2b0c76ca040c04344b
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox