STYLE: EMSegment: Fix 'Tag already registered' warnings.

This commit updates EMSegment and vtkMRMLScene::CopyRegisteredNodesToScene
to avoid warnings like this one:

//--------
Warning: In /home/jcfr/Projects/Slicer/Libs/MRML/Core/vtkMRMLScene.cxx, line 517
vtkMRMLScene (0x2590e90): Tag LinearTransform has already been registered, unregistering previous node class vtkMRMLLinearTransformNode to register vtkMRMLLinearTransformNode
//--------

Note that calling "GetClassNameByTag" in "CopyRegisteredNodesToScene"
changes "CopyRegisteredNodesToScene" complexity from O(n) to O(n^2).

Considering that

(1) "CopyRegisteredNodesToScene" is only called from
"GetReferencedSubScene", itself used only in EMSegment,

and

(2) testing times associated with EMSegment tests does not
increase

.. removing the warning to facilitate the understanding of the test outputs
justifies the added complexity.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25184 3bd1e089-480b-0410-8dfb-8563597acbee