title: SCNNode direction issue
body: <p>I put a <code>SCNNode</code> to a scene. I want to provide proper rotation in space, because this node is a pyramid. I want <code>Z axis</code> to be pointed to <code>V2</code> point, and <code>X axis</code> to be pointed to <code>V1</code> point (these V2 and V1 point are calculated dynamically, and of course in this case the angle between axis will be 90 degrees, because I calculate them properly).</p>
  
  <p><strong>The problem</strong>: I can't point X axis, because <code>SCNLookAtConstraint(target: nodeWithV2)</code> points only Z axis, and what I see is that Z axis is OK, but X axis is always random, that's why my Pyramid orientation is always wrong. How can I point X axis?</p>
  
  <p>Here is my code:</p>
  
  <pre><code>let pyramidGeometry = SCNPyramid(width: 0.1, height: 0.2, length: 0.001)
  pyramidGeometry.firstMaterial?.diffuse.contents = UIColor.white
  pyramidGeometry.firstMaterial?.lightingModel = .constant
  pyramidGeometry.firstMaterial?.isDoubleSided = true
  let nodePyramid = SCNNode(geometry: pyramidGeometry)
  nodePyramid.position = SCNVector3(2, 1, 2)
  parent.addChildNode(nodePyramid)
  let nodeZToLookAt = SCNNode()
  nodeZToLookAt.position = v2
  parent.addChildNode(nodeZToLookAt)
  let constraint = SCNLookAtConstraint(target: nodeZToLookAt)
  nodePyramid.constraints = [constraint]
  </code></pre>
  
  <p>But that only sets the direction for Z axis, that's why X axis is always random, so the rotation is always random. How can I set X axis direction to my point V1?</p>