Implementing quick sky subtraction

[ajmejia]

Adding two new functions for quick sky subtraction:

• sky spectra interpolation at science/standard fiber positions (interpolate_sky) using a spline
• quick sky subtraction by combining sky fibers from each telescope in a on-sky distance weighted sum (quick_sky_subtraction)
• updating create_fiberflat and apply_fiberflat to play with the original wavelength grid
• updating quick DRP to subtract sky before wavelength resampling
• implementing flux conserving interpolation method in Spectrum1D class
• changing dtypes from double to float