Repair broken cells

The cleaver tet mesh generation sometimes produces points with NaN values.
These points are no good when they are evaluated with the weights or when
used in SOFA.
In order to patch the mesh (while waiting for Cleaver to be fixed), we
make sure that every cell has valid points. To prevent overhead, the lookup
is done when the point and cells are transfered from  cleaver to vtk.
The patching relies on the vtkCenterOfMass.
Fix not-padded mesh transformations

The trick is that ITK's points are in the center of the voxel whereas the
mesh obtained starts from the corner of it. This means that an offset of
half the diagonale of a voxel is always needed to align the mesh with the
image (regardless of paddding or not).
Also rename the voxelDiagonale to a more explicit name to facilitate
comprehension.