ENH: Moved up vtkMRMLStorableNode is the MRML node hierarchy
vtkMRMLStorableNode is not a children of vtkMRMLTransformable node anymore, but directly a children of vtkMRMLNode.
This allows making a node storable without requiring it to be also transformable. It is important for several node types (color maps, tables, etc), which require separate storage node but not transformable.