Keep the input label value for the mesh material

The output tetrahedral mesh obtained has different material property that
depend on the label maps given to it. As of before, these labels where
given by the mesher, thus making it hard to identify the structural
correspondences in the mesh and the input labelmap.
This is done by going through the relabel map and the input image and
mapping the correspondences between the label.
Now, the material array of the output is controled by the input.
WIP Actual padding of the input images

The mesher can have some difficulties when the mesh data is too close to
its boundaries, Adding some background padding to the image can solve that.
Previously, the padding was commented, now we add it after the relabel
filter as we are sure that the background will have the value 0. The
extra padding value of 4 pixel was chosen arbitrarily.