Allow specifying the number of input channels

[makivlach]

I would find it convenient for my use-case to have an option to specify the number of input channels. Perhaps others would appreciate it too.

[MarkKremer]

Hi 👋,

I completely agree. The reason I didn't include this parameter when I wrote this is because Beep uses a [][2]float64 regardless of the channel count specified in the format.

Three improvements before I merge:

• The size of the buffer used by Streamer needs to match the channel count.
• In the Stream function the variable-channel buffer needs to be mapped to the 2-channel samples that beep expects.
• OpenDefaultStream should return an error if more than 2 channels are used. Because that doesn't work with the Beep format.

[makivlach]

Hi @MarkKremer!

Thank you for your feedback! I've finished the changes as you've requested. I hope I didn't forget anything. If you feel like changing anything further, let me know!

Cheers!

Edit: sorry, my brain is being wholly drained today. I've additionally changed the function for converting buffer into samples to work immutably.

[MarkKremer]

LGTM, thanks for the PR!