OPT: changed plan_ref for dealing with negative values and changed cubic-to-point to treat group of labels by value
OPT: changed plan_ref for dealing with negative values and changed cubic-to-point to treat group of labels by value

Former-commit-id: cd1ba46c7180fd929679b042de96698044bdb6f6
Former-commit-id: 33cd5b09b46508a813655252a633dd66a110cb69
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox