OPT: msct_smooth: new function to compute 2D derivatives (used by straighten_spinalcord)
OPT: msct_smooth: new function to compute 2D derivatives (used by straighten_spinalcord)

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