Open the atlas db.
    Return a connection.
    Return None if it doesn't exist

def atlasdb_open( path ):
    """
    Open the atlas db.
    Return a connection.
    Return None if it doesn't exist
    """
    if not os.path.exists(path):
        log.debug("Atlas DB doesn't exist at %s" % path)
        return None

    con = sqlite3.connect( path, isolation_level=None )
    con.row_factory = atlasdb_row_factory
    return con