Map SSMs to chromosomal locations.

def _read_ssm_locs(in_file):
    """Map SSMs to chromosomal locations.
    """
    out = {}
    with open(in_file) as in_handle:
        in_handle.readline()  # header
        for line in in_handle:
            sid, loc = line.split()[:2]
            chrom, pos = loc.split("_")
            out[sid] = (chrom, int(pos))
    return out