Handle zoom while centering objects

[arjxn-py]

Currently if an object is too zoomed out or zoomed in it remains like that while centering objects to the scene.

Originally discussed in #506 (comment)

[arjxn-py]

Hi, I've been on it and i didn't yet find a solid way to handle it nicely. The reason being that the plan requires the object size, i tried passing the object in customEvent and then used the object in lookatPosition but since object is not a THREE.Object3D instance to get boundingSphere or box to get idea of size. And then there'll be the handling of zoom factor for two cameras.

I'd not want to come this in the way of 3.0.0 unless we have a solid plan. And this is not that significant too. Feel free to let me know your thoughts too.