Prohibit dynamic URI base for SceneJS.Instance

Dynamic config of SceneJS.Instance nodes is fine, as long as it's only the fragment part of the URI that changes.

Good:

This is OK because the Instance will loadone subgraph, once and for all, then instance different camera Symbols from within that.

SceneJS.instance(
   function(data) {

     return {
          uri: "http://fo.com/models/myModel.dae" + data.get("fragment") // Eg. "#VisualSceneNode/camera1"
     };
 })</code>

Bad:

This is bad because internally the Instance might load a completely different file each time, which although theoretically SceneJS memory management would keep up, it would do heavy swapping.

SceneJS.instance(

function(data) {
     return {
          uri: data.get("uri") // Eg. "http://fo.com/models/myModel.dae#VisualSceneNode/camera1"
     };
 })</code>