Initial outline of simple models.

[jeromekelleher]

We want easy ways to create standard models. This is a first pass for discussion. Use like

import demes

# 10 population island model, each of size 100
g1 = demes.models.island_model([100] * 10, migration_rate=0.1)
# 3 population stepping stone model, of size 10, 20 and 30
g2 = demes.models.stepping_stone([10, 20, 30], migration_rate=0.1)

Using the demes.models namespace here seems like a nice way to do this.

[grahamgower]

That looks nice and clean, and I think the demes.models namespace is a good one. It probably does belong here in Demes, but in any event I'll link to the related stdpopsim issue.

[jeromekelleher]

Great, I'll add some tests. Generic models always seemed like bloat in stdpopsim to me.