Viz tutorial section on reducing tree size

[hyanwong]

Often trees are too big to visualise properly. We should have a section about how to slim them down. using simplify, or reducing to clonal lineages (see tskit-dev/tskit#1220), or maybe lineages that are all from one population (we don't have that functionality yet)

[hyanwong]

There are examples in viz.html#simplifying-larger-plots so I think we can close this