add AedAeg demographic model

[petrelharp]

There's a good demographic model in Crawford et al:

[jeromekelleher]

This would be best implemented as a "trunk model" like this example from the msprime docs: https://tskit.dev/msprime/docs/latest/demography.html#trunk-population-models

(Although I doubt the ancestral population is accurately described as "Senegal Forest"?)