function Rate(numpan, timepan) {
      this.numPan = void 0;
      this.timePan = void 0;
      this.startTime = void 0;
      this.nextTime = void 0;
      this.numPan = Span.setSpanValue(Util.initValue(numpan, 1));
      this.timePan = Span.setSpanValue(Util.initValue(timepan, 1));
      this.startTime = 0;
      this.nextTime = 0;
      this.init();
    }
Creates a new Rate instance.
The number of particles per second emission (a [particle]/b [s]).
@param {Array|number|Span} [numpan=1] - The number of particles for each emission.
@param {Array|number|Span} [timepan=1] - The time interval between each emission.
@example
// Create a rate of 10-20 particles every 0.1-0.25 seconds
new Rate(new Span(10, 20), new Span(0.1, 0.25));