comment on message loaders notify (was message listeners before)

[fergian94]

the _playhead_positions_cvs condition variables are consumed by the message loader threads. While the member function add_listener(self,topic,listener) of the bag_timeline is the producer of these condition variables, thus sending notify signal to all the loader threads.
The comment was wrong before: "# Notify the message listeners"
The comment I suggest is instead: "# Notify the message loaders to get them ready" to explain that the loaders are notified, and since it can be misleading from a function named add_listener, I also stress the fact that it's to prepare them to load the message and, in turn, notify the listener threads.