improve recipe 17.5 by using tspop or link_ancestors

[bhaller]

Recipe 17.5 (local ancestry using tree-seq) is outdated in its techniques. See:

https://groups.google.com/d/msgid/slim-discuss/f7a2c222-660f-4a73-a43b-e16b39abdc19n%40googlegroups.com

As @petrelharp says there, tspop or link_ancestors now provide better solutions.

Of course, the recipe is also demonstrating how to do stuff in Python yourself. So perhaps we should keep the existing code, and then say "but there's a better way to do this!" and show the modern solution to the problem as well?