Add harmonic-percussive source separation

[jurihock]

Extract and transpose only the harmonic signal independently from the percussive signal.

• Harmonic/percussive separation using median filtering
• Extending harmonic-percussive separation of audio
• Demucs Music Source Separation

TODO:

• https://sigsep.github.io/open-unmix
• https://github.com/sevagh/Real-Time-HPSS
• https://github.com/sevagh/Zen
• https://github.com/tachi-hi/HPSS

[jurihock]

Thoughts:

• HPSS could be generally interesting for instrumental records (percussion separation) as well as vocal records (plosive separation).
• Especially in case of vocal records a higher quality algorithm is required, to reduce the propagation of audible artifacts.
• Single pass (and real time) processing could be problematic, since the median filter kernel size tends to be large and requires samples from the "future".