Introduce flag to list labels (#3012)
Introduce flag to list labels (#3012)

* Removed TODO

* Added flag -list-labels

* Implemented custom argparse action to bypass required argument

* Fetching info_label.txt file from -f flag and display it

* Fixed description for methods map and ml

Fixes #2894

* Removed temporary code

Made pytest fail (thank you pytest!)

* Update scripts/sct_extract_metric.py

Co-authored-by: Joshua Newton <935c8c2fd33f20b56626cb3b7147ea3501264af9@gmail.com>

* Removed useless "__init__" in _ListLabelsAction

Context: https://github.com/neuropoly/spinalcordtoolbox/pull/3012#pullrequestreview-525550375

Co-authored-by: Joshua Newton <935c8c2fd33f20b56626cb3b7147ea3501264af9@gmail.com>
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