get_snps_set
extract the SNP sets from the output of
`find_optimised_snps`.
get_snps_set(results, as = "data.frame")
output from `find_optimised_snps`
output format, either `data.frame` or `list`.
will return either 1. a dataframe containing SNPs_set (SNP position separated by ",") and score 2. a list containing SNPs_set (SNP position as numeric vector) and score (attr of the list)