Add large scale Illumination effect / Vignetting

[ShannonS00]

This effect should capture the large scale drop off of illumination over the whole detector (Vignetting).

• It is planned to be modelled as a 2D Gaussian centred on the detector, normalised so the centre = 1.0.
• A parameter parameter edge_fraction sets how bright the corners are relative to the centre.
• Application: obj.hdu.data *= illumination_map, WCS-aligned to the image plane pixel grid OR pixel scaled when detector is read out.
• Class name: LargeScaleIllumination

Open questions

• Location of file: add new directory scopesim/effects/flats/vignetting.py or add it somewhere else?
• Z-order: 750 (ImagePlane effect, applied during observe() after FOV stacking, before detector readout) OR z-order around 800 ?
• Should the effect be included by Default?
• Should we add it in default.yaml (as !OBS)

[oczoske]

• I think a top-level illumination.py is fine, can't think of a better name...
• The model is fine, but it would be good to include it as a Callable so that it can be replaced by a different model when the need arises. Not sure of the exact syntax, but the effect would be instantiated with model=_gaussian2d, modelargs={"amp": 1, "mu": (0, 0), "sigma": (2000., 2000.)} and in the code you'd use model(*modelargs) or something like that. It should also be possible to move the model around, maybe with separate parameters xc, yc (or directly in mu?).
• Applying the effect to the ImagePlane is fine for imaging but it would have to be applied to the FieldOfView for spectroscopy. I think for now imaging is enough, though, so z_order 750 seems right.
• Not included by default, because science users should not be burdened with this.
• Unless there are other types of illumination the class name can be shortened to Illumination.