Definitely work in progress.

I've liberally used TODOs throughout for places where I expect things to significantly change.

A few things that popped out.

1. Depending on how the LUT-engine abstraction played out much of the atmosphere.init may move
2. If we want a generic LUT class, we can get rid of the big lut_exist block
3. We'll need to go through each of the engines to make sure they reflect the new inheritance/config structure
4. There are a couple of keys (e.g. multipart_transmittance) that may make more sense at the forward_model level