Documentation and DspNode trait

[Hasenn]

this pr

• switches some comments to doc comments along with formatting them a little for easier reading in an IDE
• changes some ubuntu dependencies in readme.md that didn't work for me
• adds a trait for dsp nodes to allow common documentation on those, and to statically ensure we can't forget a method

pub trait DspNode {
    /// number of outputs this node supports
    fn outputs() -> usize;

    fn set_sample_rate(&mut self, _srate: f32);

    fn reset(&mut self);

    fn process<T: NodeAudioContext>(
        &mut self, ctx: &mut T, _atoms: &[SAtom], _params: &[ProcBuf],
        inputs: &[ProcBuf], outputs: &mut [ProcBuf]);
}