Non uniform sky

[julienguy]

Implemented several routines to fit a non-uniform sky.
See issue #368 for details.

[sbailey]

Tested using the 17.12 reference run data that does not have sky variation over the focal plane. Comparing to the "truth" with no angular / chromatic variations that takes 5 seconds to run:

• sky.compute_sky(frame, angular_variation_deg=2) takes 1 minute is consistent to 0.1%
• sky.compute_sky(frame, angular_variation_deg=2, chromatic_variation_deg=2) also takes 1 minutes and is consistent to 0.2%
• sky.compute_sky(frame, angular_variation_deg=2, chromatic_variation_deg=-1) (i.e. each wavelength is separately solved) takes 15 minutes and is only consistent to 10%. Let's not use this one... :)

We could go ahead and merge this as work in the right direction; thanks for keeping the various options separated into different functions for comparison. For real use though we may need to simultaneously solve all 10 petals with consistency constraints along the boundaries. Of course the different spectrographs themselves will have differences in the uncalibrated sky due to throughput variations, but I'm wondering if the higher order terms should be constrained to be continuous. e.g. here are the 10 different independent sky solutions with angular_variation_deg=2, shown relative to the solution without any extra degrees of freedom:

[sbailey]

Merging now in the spirit of directionally correct progress that doesn't break backwards compatibility / performance. We can continue to update this with new branches and PRs even if this non-uniform sky isn't ready to be default yet.