Model updates

* Replaced `Model.computeWorldBoundingSphere` with `Model.boundingSphere`.
* Added `Model.minimumPixelSize` property so models remain visible when the viewer zooms out.
* Added ability to modify glTF material parameters using `Model.getMaterial`, `ModelMaterial`, and `ModelMesh.material`.
* Added `asynchronous` and `ready` properties to `Model`.
* Added `Cartesian4.fromColor` and `Color.fromCartesian4`.
* Added `getScale` and `getMaximumScale` to `Matrix2`, `Matrix3`, and `Matrix4`.