title: Swift SKPhysicsBody empty subclass doesn't inherit parent inits
body: <p>While trying to inherit from SkPhysicsBody I found something rather strange : 
  None of the convenience init are inherited!</p>
  
  <p>Here is the code you can test in a playground : </p>
  
  <pre><code>import PlaygroundSupport
  import SpriteKit
  import GameplayKit
  
  public class SKPhysicsBodySubClass:SKPhysicsBody
  {
  
  }
  
  let pb = SKPhysicsBody(circleOfRadius: 10)
  let pbsc = SKPhysicsBodySubClass(circleOfRadius: 10)
  </code></pre>
  
  <p>The last instruction produce an error suggesting that the parent init doesn't exist : "Incorrect argument label in call (have 'circleOfRadius:', expected 'coder:')"</p>
  
  <p>I'm also aware of the rule of inheritance : </p>
  
  <blockquote>
    <p>Rule 1 : 
    If your subclass doesn’t define any designated initializers, it automatically inherits all of its superclass designated initializers.</p>
    
    <p>Rule 2 : 
    If your subclass provides an implementation of all of its superclass designated initializers—either by inheriting them as per rule 1, or by providing a custom implementation as part of its definition—then it automatically inherits all of the superclass convenience initializers.</p>
  </blockquote>
  
  <p>Any idea what's going on here? what did I miss?</p>