ENH: Simplify MRMLReferences using smart pointers

• Use of smart pointer in vtkMRMLReference allows us to remove
  the complicated code manually managing the calls to Register/UnRegister

• function "SetNthNodeReferenceID" and "SetAndObserveNthNodeReferenceID"
  have been updated to have similar code (at the exception of the
  code setting the observation events). The second will consist in factoring
  code common code to avoid code duplication.

• SetNthNodeReferenceID/SetAndObserveNthNodeReferenceID have also
  been fixed to work properly in case the reference doesn't exist
  and referencedNodeID is null.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23191 3bd1e089-480b-0410-8dfb-8563597acbee