Add a viz notebook

[hyanwong]

I only put one plot in here, for reasons of space. But I can easily include another. For example, I could use a cut down version of this, to show how larger tree sequences can be plotted:

Or even try to incorporate a graph-like plot into tsviz so that we can claim to have an ARG viz:

[petrelharp]

This looks great! I can't do a detailed look right now, but I've added you as a collab, so feel free to merge or keep things here as you like.