Implementing reduction sequence for twilights

[ajmejia]

This branch implements the full reduction sequence of the twilight sequence in the following way:

• It runs the preprocessing and image detrending
• Extracts the fibers using fiber profile fitting
• Wavelength calibrates and rectifies
• Stacks each individual exposure spectrograph wise
• Fits each fiber continuum with a spline to remove solar spectrum absorption features and color factor
• Combines each exposure (camera wise) into a master RSS
• Resamples back to native pixel scale and stores final fiberflat

The reduction of the twillight sequence is run as follows:

drp twilight -e EXP1,EXP2,...,EXPN

or

drp twilight -e EXP1,EXPN

to expand exposure numbers into a list [EXP1,EXP2,...,EXPN]. You can also pass multiple exposure number ranges

drp twilight -e EXP1,EXPN -e EXP1,EXPM

See drp twilight --help for more options

During the reduction of twilights some QA plots are produced, namely: *twilight_continuum_fit, *twilight_flatfielded and *twilight_fiberflat_fit; to show the iterative continuum fitting, the resulting flatfielded twilight and the fiberflat, respectively.

[ajmejia]

There's still a remaining 3-5% systematic error when flatfielding twilight exposures using the master fiberflats created. This is shown in the following plot:

These systematic errors seem to be correlated with the mean count level as shown in this plot:

Where the colors mean the same as in the previous plot. We will fix this issue in a different PR.