Demucs multi-threading + 6-source (guitar, piano stems)

This release adds some great new features:

• Multi-threaded Demucs for much faster processing!
  • MAX MEMORY setting (use 16 GB or 32 GB to run Demucs faster with multiple parallel threads/workers)
  • Each worker consumes 4 GB of memory
• Make Demucs 4-source model the default; drop/deprecate Open-Unmix/UMX (you can still check the same umx.cpp code here)
• Add Demucs 6-source model, with experimental piano and guitar stems! 🎹 🎸