REF: now allow for crosses creation without reading an image in sct_label_utils
REF: now allow for crosses creation without reading an image in sct_label_utils

Former-commit-id: 54a08cc7e2c913358b1aef9bf8f86cec8285931d
Former-commit-id: 7b61a92e363723807882c578b479d2958f688643
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox