Pass identifier string when creating callout node

[francisjervis]

Adds .identifier property to Annotation and stores it as the .name property of the associated SCNNode, to facilitate handling user interactions with callout nodes.

[francisjervis]

Sample code for retrieving this in the AR VC:

override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {

            if let p = touches.first?.location(in: self.cameraView) {

                let q = self.cameraView.hitTest(p, options: nil)

                if q.count > 0 {
                    let t = q[0]
                    let n = t.node
                    if let numberTapped = n.name {

//                      respond to the interaction

                    }
                    

                }

            }

        }