Add ``store_pedigree`` option to the DTWF simulator

[jeromekelleher]

In #1858 we added the sim_pedigree function, which is a forwards-time WF simulator. It would be useful to be able to simulate pedigrees under different demographies, etc but implementing all this would be a bit of a slog.

Would it make sense to add a store_pedigree option to the DTWF simulator? This shouldn't be too hard to implement since we're already tracking individuals generation-by-generation. However, there would probably be some subtle problems where we're biasing the individuals that we store in the pedigree towards those that happen to carry genetic material from the samples.

Any thoughts?