Dithering and nodding

This is a very exciting project! I have a suggestion regarding dithering (sub-pixel offsets) and nodding (large offsets to improve background subtraction). It would be nice if these modes were natively supported and did not require the data to be resampled one extra time. A clean way to do this is to include the spatial offset of each frame in the rectification solution. Ideally, both wavelength calibration and spatial rectification should be 2D mappings from observed pixel coordinates (x,y) to final rectified coordinates (wavelength, actual "vertical" position on the sky in arcsec). This way the 2D spectrum needs to be resampled only once throughout the data reduction. I wrote a detailed description of this workflow here: https://ui.adsabs.harvard.edu/abs/2018MNRAS.478.2097B -- this may be useful for inspiration, particularly for near-infrared data (but the associated code won't be as useful as it was written in IDL and not in an object-oriented way).

Start with the issue's linked 2018 MNRAS workflow description and compare it with the current spectroscopic reduction architecture. Define how dithering and nodding offsets would be represented in the rectification solution as 2D mappings. Done would mean both wavelength calibration and spatial rectification support these offsets without an extra resampling step.

Written by the indexing model from the issue text.