PLUCK-M, MIDI 4-voice Karplus-Strong string pluck for GRAINS

I have added PLUCK-M to my GRAINS repository. PLUCK-M is a MIDI-based version of PLUCK.  Thus it is a 4-voice Karplus-Strong string plucking oscillator.  PLUCK-M has much better pitch control than PLUCK and better use of pots, but like PLUCK it cannot stay in tune for high notes because Karplus-Strong arrays are integer lengths. Still pretty good.