ind_to_fam: Convert an Eigenstrat IND tibble into a Plink FAM tibble
Description
This function takes an existing IND tibble and creates a FAM tibble with the same information and dummy values for missing data.
In particular, the output FAM tibble will contain these columns with these contents
(IND only contain id, sex, and label, so there is no loss of information):
fam: IND label
id: IND id
pat: 0 (missing paternal ID)
mat: 0 (missing maternal ID)
sex: IND sex converted to Plink integer codes via sex_to_int()