Added a basic FM function to Triceratops

The third oscillator can now become a modulator for use in FM synthesis, in FM mode the audio from the 3rd oscillator is not heard but diverted to the pitch of oscillators 1 and 2 instead, and the 3rd oscillators volume follows the filter's ADSR envelope. This produces effects similar to cross-modulation on some more advanced analogue synthesizers (e.g. Sequential circuits - Pro One.)