Implement optimal 2D background subtraction

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Algorithms exist to optimally subtract the sky background from 2D spectra that account for optical distortions and other effects. These effects create non-linear relations between spatial/wavelength coordinates and pixel-based detector coordinates. Careful mapping of these effects allow one to build a sky background model with sub-pixel resolution. One such method is described in Kelson (2003). That algorithm should be implemented in specreduce and possible updates explored.

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#193 is the first iteration of this.