ENH: serialise singleton tags

Singleton nodes were not having their singleton tag serialised to MRML,
so on loading a scene back in, two nodes would be present in the scene
if the node logic is relying on the tag to update the originally in
the scene node.
This fix serialised the singleton tag to MRML, adds a utility method
to uniformly turn the property on or off, adds a test, and expands
the node documentation to note that the hack in the scene to get
untagged singleton nodes is needed for legacy scene files. Also
gave examples for the conventions for singleton tag strings
in the mrml node header file.

Issue 0004006

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24582 3bd1e089-480b-0410-8dfb-8563597acbee