Clunky code needed to load and use a Model

I have code that needs to access model.boundingSphere as soon as the model is created. For some reason `Model.fromGltfAsync` resolves before the model.boundingSphere is "ready" to be accessed. This means that in order to do what I need, I have to always subscribe to the ready event and then immediately unsubscribe when it fires. The code looks something like this:

```js
async function loadAndZoom(url, scene) {
const doSomethingWithBoundingSphere = (model) => {
// model.boundingSphere
};

const model = await Model.fromGltfAsync({
url: url,
scene: scene
});

if (model.ready) {
doSomethingWithBoundingSphere();
} else {
const unSub = model.readyEvent.addEventListener(() => {
unSub();
doSomethingWithBoundingSphere(model);
});
}
}
```

I'm not sure why we can't access `boundingSphere` after the async factor function resolves. If that's possible, great! Otherwise if there are technical reasons that can't happen I think we need to rethink the API here.

Start by tracing Model.fromGltfAsync, model.ready, readyEvent, and boundingSphere in the Model implementation and its existing tests. Determine whether the async factory can guarantee boundingSphere availability; done means either that access works immediately after resolution or the API behavior is clearly redesigned and covered by tests.

Written by the indexing model from the issue text.