Add 'multiply_stereo_volume' FX; rename 'volumex' to 'multiply_volume'

[mondeja]

This implements the FX that controls the volume level in two channels for stereo, but also works for more channels: mono (volume control), or more than 2 channels, in this case works properly if first channel of every pair is the left.

• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it
• I have formatted my code using black -t py36

[tburrows13]

This is great. A couple of thoughts:
I don't really like the name audio_volume_lr. I'd prefer not to use l and r in place of left and right. I can't think of a good name yet, so maybe you had some more ideas? audio_left_right isn't great either. audio_adjust_stereo?

Would you like to add a changelog entry? If not I'll do it before merging.

[mondeja]

I don't really like the name audio_volume_lr. I'd prefer not to use l and r in place of left and right.

Yes, I was thinking the same, but audio_left_right seems too much general to me, I prefer audio_adjust_stereo, but this could be confused with panning. Maybe audio_stereo_volume, or audio_volume_stereo, or audio_volume_left_right?

Would you like to add a changelog entry?

I will add it 👍

[coveralls]

Coverage increased (+0.2%) to 66.159% when pulling 3d3464e on mondeja:audio-volume-left-right into 727276f on Zulko:master.