ENH: Add volumetric mesh support in vtkMRMLModelNode

See: https://www.na-mic.org/Wiki/index.php/2017_Winter_Project_Week/Support_for_volumetric_meshes

Update vtkMRMLModelNode to store a vtkPointSet instead of a
vtkPolyData. vtkPointSet can be a base class for vtkPolyData
(surface mesh) as well as vtkUnstructuredGrid (volumetric mesh).

The safest way to know if the mesh is unstructuredGrid is to
check if vtkMRMLModelNode::GetUnstructuredGrid() is not NULL,
but since it requires to update the pipeline, vtkMRMLModelNode
also has a member variable called MeshTypeHint.

For backward compatibility, keep the public polydata-based API
and wrap around the mesh-based API in vtkMRMLModelNode and
vtkMRMLModelDisplayNode.

Update vtkMRMLModelDisplayNode, vtkMRMLModelDisplayableManager,
vtkMRMLModelSliceDisplayableManager, and qMRMLModelDisplayNodeWidget
to use the mesh-based API by default.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25660 3bd1e089-480b-0410-8dfb-8563597acbee