Second attempt at adding demucs for instrumentals

[bohning]

Following bobobo1618's suggestion in #168 , this PR implements stem separation using demucs-cxfreeze as external dependency.

Works like a charm (albeit slow) on my intel Mac with the respective demucs-cxfreeze binaries.

[randompersona1]

We're telling users to download an unknown executable from a repository we don't control. With all due respect to the author of demucs-cxfreeze, I would prefer building ourselves, which probably means creating a new repository.

This would have another advantage, because we could wrap demucs in our own cli, which would enable us to switch in the future to a different splitting model without changing the syncer.

As I mentioned already as well, this definitely needs some kind of synchronous queuing to not overload user systems. Even two or three at once is tough on the CPU.

Note that I haven't really had a look at the actual changes yet.

[bohning]

Agree to all of it.

As I mentioned already as well, this definitely needs some kind of synchronous queuing to not overload user systems. Even two or three at once is tough on the CPU.

For my machine, already one is tough on the CPU :-D