Organic: make mixing of oscillators independent of lower oscs

Closes #590, but I can't say if there should be some compatibility code added to it. Might be wise, as the bug has probably been there for years... 

The issue was that the ```m_ModulationAlgo``` IntModel was constructed with ```default=Oscillator::SignalMix, min=0, max=0``` which forced the value to 0.