NEW: small function to calculate bvalue
NEW: small function to calculate bvalue

Former-commit-id: 5c8c9220e95e7f8b554d55cc3b0093e388199464
Former-commit-id: 0451ee7bbed40376a1d6908f737225229569c757
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox