STYLE: Use static member variables for reference roles and attribute name

Issue #2727

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22186 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Copy NodeReferenceMRMLAttributeNames in vtkMRMLNode::Copy()

Issue #2727

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22187 3bd1e089-480b-0410-8dfb-8563597acbee