Map sequence references to snap reference directory, using standard layout.

def remap_index_fn(ref_file):
    """Map sequence references to snap reference directory, using standard layout.
    """
    snap_dir = os.path.join(os.path.dirname(ref_file), os.pardir, "snap")
    assert os.path.exists(snap_dir) and os.path.isdir(snap_dir), snap_dir
    return snap_dir