This function treats the data as diploid; you can divide by 2, and then treat 0.5 as NA, if it's actually haploid.
ReadIntList(pgen, variant_subset)
Integer matrix, where rows correspond to samples, columns correspond to variant_subset, and values are in {0, 1, 2, NA} indicating the number of hardcall ALT allele copies. For multiallelic variants, all ALT alleles are combined.
Object returned by NewPgen().
Integer vector containing 1-based indexes of variants to load.