add twilight sky model

[sbailey]

The current specsim sky model is based only upon lunar parameters so if the moon is down during twilight, the sky gets simulated as if it were completely dark.

For API, nominally I'd suggest something like

simulator.atmosphere.sun_altitude = -25 * u.deg

where the default value would be some negative number equivalent to the post-twilight current airglow+moon models. sun_altitude>0 should probably generate an exception (or even sun_altitude>-10 * u.deg)

Some papers for consideration:

• http://www.eso.org/~fpatat/science/skybright/twilight.pdf
• http://adsabs.harvard.edu/abs/2016SPIE.9910E..1AY