OPT: maths: now dilate and erode can be applied to non-binary data
OPT: maths: now dilate and erode can be applied to non-binary data

Former-commit-id: 0c869a3e00ec4ce6e2f5c9b29d632a676aaca059
Former-commit-id: 03e5c13d7aecaafc222ac60a6cc97a9e37af0265
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox