Implement a model of twilight sky brightness

[dkirkby]

We now have a functional twilight model that can be used together with the scattered moon model for more realistic bright-time simulations. The two main questions I would like to follow up on eventually are:

• Is the spatial distribution of the predicted twilight brightness with the sun 13-15 deg below the horizon correct?
• How should extinction affect the twilight spectrum (currently it doesn't at all).

However, I think the general features of this model are a big improvement over nothing so should be merged soon unless anyone spots obvious problems.

This PR also removes the (now obsolete) sky-conditions config parameter, which was originally to support using the gray/bright spectra in desimodel but is now redundant with the more flexible moon and twilight models.

[dkirkby]

Update on the two issues mentioned earlier:

• @segasai has refit the SDSS data with a new spatial model. I'm not convinced that the new model is an improvement, but at least we know there is no easy fix here.
• I have implemented first-order scattering (using the extinction curve) of the solar spectrum, similar to what I already implemented for scattered moonlight.

Three new plots to document the new model (which are now in the docs, along with instructions to reproduce them):

[weaverba137]

You'll want to bring in the latest changes from master to get tests to pass.

[moustakas]

@dkirkby It would be very helpful to have this updated sky model implemented to carry out some bright-time redshift success tests, even if there's still work to be done. Can you provide any updates on when we might be able to merge this? Thanks!

[moustakas]

Any chance of getting these changes into the 18.12 software release?

[weaverba137]

@dkirkby, this PR is now about 3 years old. Should we continue to keep this open?