playback_lsl errors

[cboulay]

Grouping a couple errors together so I only have to create 1 PR.

Problem 1:

Since e8817c0, if the sleep duration (default 5 msec) is shorter than the time between two samples in any of the streams, stop_idx will not be larger than start_idx and all_streams_exhausted will remain True, causing the loop to exit and terminate playback prematurely.

I think the solution is for the streams-exhausted check to be based on (1) looping disabled and (2) the streams' time ranges. This will probably require some new state variables.

Problem 2:

From the playback entry point, one of the arguments to LSLPlaybackClock is loop_time=wrap_dur if loop else None, and this value gets assigned to self._boundary. However, the clock's t0 property tries to do self._n_loop * self._boundary which doesn't work when loop is False and therefore self._boundary is None.

The easy solution is to fix the argument to be loop_time=wrap_dur if loop else 0.0, which is probably the correct solution (vs fixing t0 property) because the loop_time annotation is non-optional float.