Wavelength correction using sky emission lines

[PabloCorcho]

Implement a new Correction that performs additional wavelength calibration steps using sky emission lines.

This method would need a model containing at least the position (ideally the instrinsic FWHM and relative flux would be also useful) of the sky lines (e.g. UVES sky lines atlas). We have to decide where to put that data / how to download it, or leave it as an input argument.