'Try to download the torrent file without a known source. A possible source could be the DHT.
If the torrent is received successfully, the user_callback method is called with the infohash as first
and the contents of the torrent file (bencoded dict) as second parameter. If the torrent could not
be obtained, the callback is not called. The torrent will have been added to the TorrentDBHandler (if enabled)
at the time of the call.
:param infohash: the infohash of the torrent
:param user_callback: a function adhering to the above spec
:param priority: the priority of this download'

def download_torrentfile(self, infohash=None, user_callback=None, priority=0):

    if (not self.lm.rtorrent_handler):
        raise OperationNotEnabledByConfigurationException()
    self.lm.rtorrent_handler.download_torrent(None, infohash, user_callback=user_callback, priority=priority)
