Get marker rsids (reference SNP cluster IDs) from a specified genome region based on markers present in `Glist`.
getMarkers(Glist = NULL, chr = NULL, region = NULL)
A vector of rsids that fall within the specified region on the given chromosome.
A list structure with information about genotypes stored on disk.
A chromosome from which markers are extracted.
A genome region (in base pairs) from which markers are extracted.