Update population diagrams in demography tutorial

[gtsambos]

The labels of all diagrams need to be updated, and some new ones need to be made for admixture + divergence.

[hyanwong]

It would be really nice if we could somehow use all the effort that @grahamgower has put in to the demesdraw visualiser to create diagrams, instead of drawing them by hand. Graham: I'm assuming that diagrams such as those in the demography tutorial would be easy to show as "tubes"?

[grahamgower]

That's absolutely doable. But maybe demesdraw is too quirky or lacks some desirable features? For example, it doesn't consider the migration rate value when drawing migrations -- all lines are equally thick. And the mgiration lines are drawn at random times, not at fixed time intervals.

[hyanwong]

The tut is thanks to @gtsambos , so I'll let her weigh in, but I thought it would be useful to point out the possibilities here. I guess she can take it up with you, @grahamgower , if there's additional functionality that would be helpful? I just really like the demesdraw stuff (and I think I should also note in my tskit viz tut PR that demesdraw exists).