Updates to fitting for faint traces. #35
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This PR updates our trace fitting algorithm to better deal with faint (read low signal-to-noise) traces. To fit the center, we take 25 pixel steps and fit the entire 25-pixel wide block using a match filter to get the trace center. We have also included error bars in the centroids estimated from this. These steps are fit to a Legendre polynomial to get initial coefficient guesses. We then fit the entire trace simultaneously (with a fixed width) to get the centers of the trace. Once we have the centers, we fix them and allow the width to vary. We include a background term that is a Legendre polynomial but the coefficients vary smoothly across the image.