Controlling max num oscs for interp partials piano

The additive-synthesis piano (patch 256) sounds better when it uses more harmonics. But it also uses more resources. I recently upped its oscillator use from 20 to 24 (by fixing an old bug) and now we can't play as many simultaneous notes on Tulip hardware as we used to. 

We should give the user control over the sound-quality-polyphony trade off by exposing a max num partials control.