New modes for Moog filter

I’ve added some selectable filter models to the MoogFilter module.

Sofar there are Huovilainen (that was the one originally implemented, and it still is the default filter), Improved, Krajeski, MicroTracker, MusicDSP, OberheimVar, RKSimulation and Stilson.

The names do not imply certain associated qualities. For more information about the filter types see the link below.

These are all from the ddiakopoulos / MoogLadders GitHub site.