Add PadImage filter
Add padding in CreateTetrahedralMesh as optional
Add PadImage filter into FEM workflow
Restore vtkMRMLDisplayableNode::GetNthDisplayNodeByClass

Used by the volume rendering section in the workflow module.
Fix margins and default properties in workflow
Merge branch 'pad-image-filter' into next

* pad-image-filter:
  Fix margins and default properties in workflow
  Restore vtkMRMLDisplayableNode::GetNthDisplayNodeByClass
  Add PadImage filter into FEM workflow
  Add padding in CreateTetrahedralMesh as optional
  Add PadImage filter

Conflicts:
	CMake/Superbuild/External_Slicer.cmake
	Modules/Scripted/Workflow/Workflow.py
Re-organize cleaver to vtk conversion

Instead of going twice over the input data to add the cells then the vertex
to the vtk arrays, we can do it in only one loop.
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.