ENH: Added vtkMRMLLabelMapVolumeNode for storing labelmaps

Before vtkMRMLScalarVolumeNode was used for both scalar and label map volumes and the LabelMap custom MRML node attribute was used for distinguishing between them (0=scalar; 1=label map volume).

Now a new vtkMRMLLabelMapVolumeNode class is used for storing label map volumes, with vtkMRMLScalarVolume used as base class.

Known limitations:
* Legacy scenes (where labelm maps is stored in vtkMRMLScalarVolumeNode) are not read correctly yet. If a volume node is read from an old scene with LabelMap attribute 1 then the node should be converted to vtkMRMLLabelMapVolumeNode.
* Automatic test results have not been analyzed yet.