individuals_alive_at( ) does not work with WF models

[petrelharp]

As discovered in #38, individuals_alive_at( ) doesn't work with WF models.

Here is a simple SLiM recipe:

initialize()
{
    setSeed(23);
    initializeTreeSeq();
    initializeMutationRate(1e-2);
    initializeMutationType("m1", 0.5, "f", -0.1);
    initializeGenomicElementType("g1", m1, 1.0);
    initializeGenomicElement(g1, 0, 99);
    initializeRecombinationRate(1e-2);
}

1 { 
    sim.addSubpop("p1", 10);
}

10 {
    sim.treeSeqOutput("remember.trees");
    catn("Done.");
    sim.simulationFinished();
}

but individuals_alive_at() does not show anyone alive at any time:

import pyslim
import numpy as np

ts = pyslim.load("remember.trees")

print(ts.num_individuals)
print([len(ts.individuals_alive_at(x)) for x in range(20)])

A look at the code reveals that the code uses individual_ages, which are all -1 in WF models. Whoops. I guess this wasn't being tested in a WF model. (damn bifurcating code paths)

One solution would be an if statement in the code, checking if it's a WF model. But I think it would make much more sense to actually set individual_ages all to 0 for a WF model, since that's actually what their ages are.

[petrelharp]

Fixed in 6121876