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

Former-commit-id: d56c7b5dade97afc0c8330e3194e1b8d6e5d4d94 [formerly 5c8c9220e95e7f8b554d55cc3b0093e388199464]
Former-commit-id: 0451ee7bbed40376a1d6908f737225229569c757
Former-commit-id: eef8ad8f3a6d71c2d72c771bb6850e0d93434398
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox