Optionnaly split the computationnal region in CreateTetrahedralMesh

When computing the a tetrahdral mesh on a full volume, the algorithm can
use a lot of memory. To remediate to this, the input image can be split
in sub regions that will each be meshed separatly.
This is controled by the NumberOfSubRegions parameter. The name of the
SaveLabelImages parameter was changed to a more generic name since it's
also used for debug polydata now. Those polydata are the mesh from each
sub-regions created by Cleaver, before any cleaning or scaling.