Load PED files
With the ability to load pedigrees from PED files, I also tried to refactor pedigree loading a bit.
Parsing code is now in separate modules and constructing the model instances is done by the file parsers instead of in the Pedigree
constructor.
Otherwise I tried to keep everything as is, but I think our models could get some more attention.
@z.tatum I'll merge if you're ok with this for now.