Interoperability tutorial #315

As tskit gains more traction, it may be worth showing how it can interoperate with other tools. This also links in with #141 .

We can simply mention that Relate and arg-needle-lib will export to tskit, but it could also be worth pointing out https://grgl.readthedocs.io/en/stable/python_api.html#pygrgl.grg_from_trees and also address quattro/linear-dag#6 (comment). It would be nice to show how to use the latter two to perform fast GWAS, which would be a win-win for all the libraries concerned, I think.

As in #240, we could create traits on top of a simulation using tstrait.