ENH: Expose vtkMRMLSceneViewNode::GetNodesByClass to python

Since std::vector<vtkMRMLNode *> is not "wrapped" by PythonQt, this commit
add an alternative method returning the nodes as a vtkCollection.

From: Nicole Aucoin <nicole@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23383 3bd1e089-480b-0410-8dfb-8563597acbee