Re-factoring sky subtraction module

[ajmejia]

Implements the following improvements into the sky subtraction module:

• Supersky (spline-)fitting is stored in output files of interpolate_sky routine
• Sky subtraction now happening at the end of the reduction (i.e., after flux calibration)
• Final sky model is being propagated into lvmCFrame
• Fixing -s flag for skipping sky subtraction (while still saving master sky)
• Adding metadata information to sky module products (still in progress)
• Add header keywords for sky subtraction (@ymamay 's and @kslong's requests)
• Store flux scaling factors in table