analysis.treemix - toytree v3.0 compatibility

[isaacovercast]

I added some try/except blocks inside the treemix tool to account for diffs between toytree v2 and v3. I got stuck in the tmx.draw_tree() method because it started manipulating tre._coords and it wasn't quickly clear to me how to get these out of a v3 toytree, so I gave up and added a check to raise an error if toytree is greater than v2, as a stop-gap for now.