consolidate `center`, `base_scale`, and `aabb_scale` settings

[ethanweber]

Right now it's very confusing to set these values when training with instant_ngp data. We should make it as easy as possible for the user to understand by consolidating these values and documenting how to use them. (This may require a refactoring of the data loading pipeline.)

[ethanweber]

~~I'll try to do this in #252~~

[ethanweber]

In instant ngp, these are "aabb_scale", "scale", and "offset" in their transforms.json file. Assigning @tancik since I know you are working on this lately.

[tancik]

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