OPT: the function cubic-to-point now operates by groups of values and not groups of volumes
OPT: the function cubic-to-point now operates by groups of values and not groups of volumes

Former-commit-id: 5ef4c971cd04e1b9042912fc9790270280971fa1
Former-commit-id: 6a143345c48b37b3717eb8b732b3144d12d2ded3
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox