Add band-limited oscillators

[ttencate]

Quoting my friend @thedjinn: "I would strongly suggest using band-limited waveforms as oscillators though, because that eliminates the audible aliasing. [...] One way to get band-limited waveforms is to create them with the Fourier series. If that's too slow (e.g. if you want real-time stuff) then precompute some wavetables and sample from these using Hermite interpolation. There are plenty of examples on MusicDSP."