REF: sct_average_data_within_mask: added parser and changed options. related to #669.
REF: sct_average_data_within_mask: added parser and changed options. related to #669.

Former-commit-id: 3cadb58c0c8cd17350fcd483d8abf27e9c59ca75
Former-commit-id: b9fe4cabb284a760b02a4f8c6e97b2ba6a5500c4
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox