Support pedigreed populations

[lvclark]

I'd like to make a new pipeline that uses pedigree information. Genotype estimates of parents and offspring will iteratively influence genotype priors of parents and offspring. Even for biparental populations, this could perform much better than the existing pipeline, which doesn't handle segregation distortion well.

I probably won't tackle this until I have added support for multiploid populations (Issue #17), in order to avoid having to rewrite a lot of code after the fact.

If you have a good test dataset for this sort of population, please contact me!