Expand generic models (eg, stepping stone model)

[petrelharp]

A reviewer suggested we do this, and I don't think we have to do it before resubmitting, but it'd be a good thing to do.

[jeromekelleher]

Various generic models are being develop in msprime as part of version 1.0. Best to wait until this is released before doing the same thing here.

[grahamgower]

See msprime docs.
https://tskit.dev/msprime/docs/stable/api.html#msprime.Demography.isolated_model
https://tskit.dev/msprime/docs/stable/api.html#msprime.Demography.island_model
https://tskit.dev/msprime/docs/stable/api.html#msprime.Demography.stepping_stone_model