Support negative indexing in Tree.unrank

[jeromekelleher]

This would be useful for generating the last tree in the ordering, as discussed in #944. We'd like to be able to write

tree = tskit.Tree.unrank((-1, 0), 6)

[benjeffery]

@szhan I assume you're not coming back to this as you have more than enough to be doing. It's only a nice-to-have, so closing for now.

[hyanwong]

Punt to 1.1? Seems uncontroversial.

[benjeffery]

Ok! Sorry, I am getting too much dopamine from hitting the close button.

[jeromekelleher]

I'm not sure there's a lot of value in having issues open that no one intends to work on, vs having old issues that were closed and are still findable. Cutting things back to a manageable set of issues is an important step.

I can see this being one being more subtle that one might imagine and I'm not interested spending time on in for the foreseeable