NEW: wrapper of ComposeMultiTransform to concatenate warping fields
NEW: wrapper of WarpImageMultiTransform to apply warping fields
NEW: new method to systematically check if '/' is there (or is not) in a path name
NEW: wrapper of WarpImageMultiTransform to apply warping fields (previous commit was a mistake)
DOC: added todo
REF: modified name sct_warp_atlas2metric --> sct_warp_atlas, and improved the code (notably enable warping of spinal levels)