BUG: 2727. converted vtkMRMLNodeReference class into a proper VTK class. Converted std::string's into char *.  vtkMRMLNodeReference is now protected class inside vtkMRMLNode class.


git-svn-id: http://svn.slicer.org/Slicer4/trunk@21850 3bd1e089-480b-0410-8dfb-8563597acbee