title: Can't prevent physics field from interacting with particle system using category bit mask
body: <p>I'd like to place multiple particle systems with corresponding fields into a single scene and toggle between them. The fields should only interact with specific particle systems. I'm unable to do so using the Xcode SceneKit document builder.</p>
  <p>As per the Apple documentation:</p>
  <blockquote>
  <p>To determine whether a field affects a physics body, SceneKit performs a bitwise AND operation on the field’s category bit mask and the body’s categoryBitMask property. If the result is a nonzero value, SceneKit computes and applies the force of the field on the body. To determine whether a field affects the particles spawned by an SCNParticleSystem object, SceneKit performs the same check using the categoryBitMask property of the node containing the particle system.</p>
  <p>Use this property to create fields which affect only certain bodies in your scene. Reducing the number of bodies affected by fields can also improve simulation performance.</p>
  </blockquote>
  <p>I've created a simple scene file that contains:</p>
  <ul>
  <li>A linear gravity field. It's category bit mask set to 2 in the Node inspector.</li>
  <li>A node. It's category bit mask set to 4 in the Node inspector.</li>
  <li>A spheric particle system, inside the above node. Affected by physics fields option checked. Category bit mask left at 1 in the Node inspector.</li>
  </ul>
  <p>Doesn't matter what I set the category bit masks to. The physics field always interacts with the particle system. I've also tried setting the bit masks programmatically in a Playground to in a separate file with 3 particle systems with one field for each system with no luck. Using:</p>
  <pre><code>let effect1Mask = 0x1 &lt;&lt; 1
  let effect2Mask = 0x1 &lt;&lt; 2
  let effect3Mask = 0x1 &lt;&lt; 3
  </code></pre>