Improve unWISE Catalog background around bright galaxies

[schlafly]

The unWISE catalog produces a background model based on the median residual after subtracting point sources in a ~1x1' grid. This leads to ugly boxy residuals around galaxies with sizes comparable to 1'. We could probably eliminate this issue by excluding pixels marked as being in LSLGA ellipses from the background median filtering.

This approach carries a bit of risk. There will be more flux left on the images in the outskirts on galaxies; it's possible this could lead to more spurious sources. Alternatively, it may be reduce the number of sources found in these outskirts, since there will be fewer peaks to find after the poor background modeling is removed.

[moustakas]

Related to moustakas/SGA#57.