ENH: Use vtkMRMLLabelMapVolumeNode class for labelmap volumes

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). This made conversion between labelmap/scalar volumes very easy but made it difficult to customize behavior, display, processing of segmentation information.

Now a new vtkMRMLLabelMapVolumeNode class is used for storing segmentation information (still using vtkMRMLScalarVolume used as base class for backward compatibility; but in the future the base class may be changed to reflect that segmentation can be represented in various ways, not just as volumes).

Notes:

A new "segmentation" node has been developed that is based on vtkMRMLLabelMapVolumeNode, which can store segmentation information in various ways (not just labelmaps but planar contours, closed surfaces, etc). To allow using of this new segmentation node, first the vtkMRMLLabelMapVolumeNode has to be integrated into the Slicer core.

When reading a scene that was saved with an old version of Slicer, label maps are stored in vtkMRMLScalarVolumeNode with label map attribute set to 1. Added a check for this case and handle it by converting the vtkMRMLScalarVolumeNode to a vtkMRMLLabelMapVolumeNode during import.