Add a function for voxelisation of 3D mesh
We think that voxelising the 3D liver mesh will facilitate the volume computation and cutting of the liver. I found a code for voxelisation using VTK: http://vtk.1045678.n5.nabble.com/Solid-Voxelization-with-VTK-td5747520.html . It works but is in C++. I'll convert it into python. @MattClarkson: I'm wondering if it's good to put this function in /utils, or somewhere else.