Added TreeModel.IterNext()

I don't understand why, but the "files changed" for this pull request is showing every change I've made since I forked your repository, even the ones that have already been integrated into the main fork. Hopefully on your end you're able to just see the actual diff between our two branches. If not, please feel encouraged to advise this mere mortal on the correct Github pull request procedure, because the Github docs and so many Google searches have failed to instruct me.