0v3 all stops pulled

Implements 16'' 8'' 4'' II and III drawbars (inspired by Vox Continental keymap), all stops out for now.
Left some key-off leakage (...no this organ isn't broken. It's just ON)

0v2

single 8'' drawbar at 0 dBfs => more than one key at once will clip, which is fun.

0v1 alsa

functional on Raspberry Pi / Alsa.
Latency with a USB audio interface is OK-ish but too high for real-world use
Latency with the PWM audio port is too high