update SLiM script for verification.py

[GertjanBisschop]

The current SLiM scripts in verification.py throw up a few errors when run with SLiM 4.0 (default installed version is the latest version of SLiM). For example, the concept of time/generations has been modified slightly, sim.generation can no longer be called to track time.

The version of SLiM used for statistical tests needs to specified explicitly and the scripts require some minor tweaks if we go with version 4.

[jeromekelleher]

There's a makefile to download and compile SLiM, so perhaps we should just stick a particular version on that download (something < 4) and make sure that the verification.py will always use that?

[GertjanBisschop]

Yup. Just for completeness. Had a little look, the actual modifications are minor.
Explicitly add 1 early() to the event populating the simulation. And replace sim.generation with sim.cycle.

[jeromekelleher]

Great, thanks. I don't mind which direction we go.