Arecorder track selection and gain.

[wiccy46]

In feature-recorder-improvement, I added a new method to Arecorder named set_tracks(self, tracks, gains). This allows individual tracks selected to be recorded before calling record(), e.g.:

ar.set_tracks(tracks=[0, 2], gains=[-3, -10]) #gain in db

or just with one channel: `ar.set_tracks(tracks=2, gains=-10)

I then added data_float = data_float[:, self.tracks] * self.gains to the callback to actually subset the channels.

I also added to reset the recording mode back to record all track with no volume adjustment.

def reset(self):
        self.tracks = slice(None)
        self.gains = np.ones(self.channels)

Originally, I was thinking to introduce input channel selection in record() but think it is best for user to refined the desired channels first just like it would be done in a DAW.

[aleneum]

@wiccy46: Has this been solved or should that be moved to for instance 0.5.1?

[wiccy46]

This has not been resolved. And unlikely will be in 0.5.1 either. I will just remove the milestone. It is not a critical feature. So I guess it has to wait.