Sort a set of regionally split files by region for ordered output.

def _sort_by_region(fnames, regions, ref_file, config):
    """Sort a set of regionally split files by region for ordered output.
    """
    contig_order = {}
    for i, sq in enumerate(ref.file_contigs(ref_file, config)):
        contig_order[sq.name] = i
    sitems = []
    assert len(regions) == len(fnames), (regions, fnames)
    added_fnames = set([])
    for region, fname in zip(regions, fnames):
        if fname not in added_fnames:
            if isinstance(region, (list, tuple)):
                c, s, e = region
            elif isinstance(region, six.string_types) and region.find(":") >= 0:
                c, coords = region.split(":")
                s, e = [int(x) for x in coords.split("-")]
            else:
                c = region
                s, e = 0, 0
            sitems.append(((contig_order[c], s, e), c, fname))
            added_fnames.add(fname)
    sitems.sort()
    return [(x[1], x[2]) for x in sitems]