Refined wavelength solution using stack of measured sky spectra

[julienguy]

• Develop a code to measure systematic errors in wavelength solution (per fiber) on edges of sensitive range of each CCD using many spectra from sky exposures (need an external sky spectrum with accurate wavelength calibration).
• Save this is in a calibration data file TBD.
• Use measurements as a prior on high order coefficients of Legendre polynomials of wavelength solution in the fit of arc lines (in specex).

[profxj]

I have code in the flexure branch that has a fair bit of this functionality. Will aim to push it to a PR by the end of the week.