Rethink BT SETTL models

[kelle]

Currently, the BT SETTL models are being distributed as part of this package and resulting in it being quite large. We should figure out a different way of distributing these files.

[kelle]

Actually, this atmospheric modeling aspect of the package seems "extra" and not needed for the core functionality. Maybe we can just remove the BT SETTL models? Or include a sparse grid and then include documentation about how to add any desired model grids?

[hover2pi]

We could also move all the data files to a required download to make the application lighter. We do this with most of STScI's data-heavy applications.

[kelle]

Since both Jackie and Sherelyn gave the green light to remove them (on Slack), I'm going that route at the moment. I'm just deleting the btsettl/ directory and leaving the rest.

[kelle]

Removed in #126 but keeping this issue open in case we want to re-visit.