Animating the SPR operation in PyGame

[brianzhang01]

Right now going between marginal trees just draws a fresh tree. It would be great to add an animation of the process -- called an SPR for subtree pruning and regrafting. I already have some ideas on how to show this, but making it cover all the cases may require some casework of what sorts of SPRs can possibly show up. We can make the animation speed selectable by the user in a set of options on the side of the UI, with an option to turn animations off.

[jeromekelleher]

Sounds great @brianzhang01. It might be worth thinking about the full-arg case here as well, as it makes these SPRs easier to see. You can get these from an msprime simulation with record_full_arg=True.