A couple of small changes to internal node labelling

[jergosh]

Hi Asif,
I had a go at fixing two small issues with labelling internal nodes I've encountered while running tdg09 on two fairly messy trees with a lot of group switches. I first noticed that tree labelling was crashing if the name of a group was longer than two characters and added a simple check for this.

This helped with one tree but labelling still crashed on the other. I think the issue was caused by parent-child node pairs that were both group switch ("_GS") nodes. In such cases, children nodes are labelled first but this was failing as the labels are set based on the group of the parent (which is at that point unlabelled). I think simply iterating over unknownNodes in reverse ensures the right order but I haven't tested this change extensively.