Add material to the basic workbook

[hyanwong]

Hi @benjeffery - the basic tskit workbook was looking sparse, so I prettified it up a bit. Do you think (a) this is the right approach (it's quite plot-heavy, targetted at end-users rather than programmers) and (b) there's the right amount of content there?

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[hyanwong]

So that I could get a nice PCA plot, I made a new tree sequence, generated from a simple island model:

import msprime

demography = msprime.Demography.island_model([1000, 1000, 1000, 1000], migration_rate=2e-5)
ts = msprime.sim_ancestry(samples={"pop_0": 25, "pop_1": 25, "pop_2": 25, "pop_3": 25},
                          recombination_rate=1e-8, sequence_length=1e6, demography=demography, random_seed=123)
ts = msprime.sim_mutations(ts, rate=1e-8, random_seed=123)

[benjeffery]

Looks great @hyanwong - I think the targeted audience is right, this is a showcase of using tskit.