BUG: 3060. Made 
const std::vector<vtkMRMLDisplayNode*>& vtkMRMLDisplayableNode::GetDisplayNodes();
 - Obsolete
 utility function that provides an unsafe API. Please use GetNumberOfDisplayNodes() and GetNthDisplayNode() instead 

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21898 3bd1e089-480b-0410-8dfb-8563597acbee