Add Demucs v3 (hdemucs_mmi) support

This project now supports inference for Demucs v3 (hdemucs_mmi) weights

Demucs v3 is considerably faster than the v4 architecture, and very close in quality to v4 in some songs

Android app

Announcing the release of my new Android app based on demucs.cpp - the power of Demucs in your pocket! Install it from the Play Store today.

Add multi-threaded Demucs

There are now example driver programs for multi-threaded demucs

The threading-related code is in cli-apps (for the driver programs), not in src (for the library). This is an intentional separation, since I don't consider threads to be a core part of the inference library, but an enhancement for end-user applications (like the cli-apps or free-music-demixer