Add 1000G pedigree, parse, unrelated sample IDs w/ data

[x] Add 1000 genomes pedigree to data-raw: http://ftp.1000genomes.ebi.ac.uk/vol1/ftp/release/20130502/integrated_call_samples_v3.20200731.ALL.ped
[x] Add code to parse this to find relatives, unrelateds, etc., e.g. using kinship2::pedigree.unrelated() or otherwise.
[x] Example code to write out lists of unrelated sample IDs having genotype data in Phase III (in a vignette?)

We'll need unrelated founders in signaturescience/skatesim#1. This will help find those from each population.

signaturescience / skater