Set scene scale

[tancik]

Viser currently assumes the scene is roughly 10x10x10 which makes navigation difficult when your scenes are a very different scale. For example, nerfstudio is +-1. Users currently scale everything on the server side, but we should add the functionality to set the scene scale in viser.

One api possibility is,

viser_server = ViserServer()
viser_server.set_scale(10)