OPT: improved speed of label removing and added the possibility to remove symmetrically on input and ref images
OPT: improved speed of label removing and added the possibility to remove symmetrically on input and ref images

3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox