title: iOS SpriteKit Emitter particle frequency
body: <p>Is it possible to change how often a particle is emitted. For example, if I have one particle emitting, can I have it emit every 5 or 10 seconds?</p>
  <p>I searched the documentation, but could not find anything. Is there a workaround? I would like to do something like this:</p>
  <pre><code>   emitter.particleBirthRate = 1
     emitter.particleBirthRateFrequency = 5  // this does not exist
  </code></pre>