NEW: new option in sct_crop_image. Possibility to crop with minus values.
NEW: new option in sct_crop_image. Possibility to crop with minus values.

Former-commit-id: 8217f51dbf31fcc988e3a8d1da2eb540338a5261
Former-commit-id: f85b5c238101d65e16463a066e5dd4400d333395
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox