BUG: Fix behavior of "Set(AndObserve)NthNodeReference" function

This commit now ensures that if a reference is set to position N,
it can later be obtain given N.

The issue is illustrated below:

SetNthNodeReferenceID("MyRole", / n= / 1, "ID2")
GetNthNodeReferenceID("MyRole", / n= / 1) => return NULL
GetNthNodeReferenceID("MyRole", / n= / 0) => return "ID2"

=> Getting reference N=1 should return ID2

For reference, the proposed change has also been discussed on the mailing
list. See [1]

[1] http://slicer-devel.65872.n3.nabble.com/MRML-References-update-proposal-tt4031425.html

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23192 3bd1e089-480b-0410-8dfb-8563597acbee