BUG: #3076 make sure scalar range is updated after edit operations.

This new method unifies the updating of the image data and volume
node to trigger rendering.  By calling modified on the point scalars,
this GetScalarRange call returns the correct data.

http://na-mic.org/Bug/view.php?id=3076

This is confirmed by looking at the results of the call after
editing in new colors as shown below.

>>> l = getNode('*label')
>>> l
(vtkMRMLScalarVolumeNode)0x131766ba8
>>> l.GetImageData().GetScalarRange()
(0.0, 15.0)
>>> l.GetImageData().GetScalarRange()
(0.0, 18.0)
>>>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21892 3bd1e089-480b-0410-8dfb-8563597acbee