filter(freq, q) {
    this._leftFilter.set(freq, q);
    this._rightFilter.set(freq, q);
  }
Set a lowpass filter frequency for the delay. A lowpass filter
 will cut off any frequencies higher than the filter frequency.

 @method  filter
 @for p5.Delay
 @param {Number|Object} cutoffFreq  A lowpass filter will cut off any
                             frequencies higher than the filter frequency.
 @param {Number|Object} res  Resonance of the filter frequency
                             cutoff, or an object (i.e. a p5.Oscillator)
                             that can be used to modulate this parameter.
                             High numbers (i.e. 15) will produce a resonance,
                             low numbers (i.e. .2) will produce a slope.