BUG: fix issue #671 (when the user asked for an unavailable vertebral level in the caudal direction --> take the highest level available and display a warning message)
BUG: fix issue #671 (when the user asked for an unavailable vertebral level in the caudal direction --> take the highest level available and display a warning message)

3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox