[NEW FEATURE] Ubersegmentation map

The sextractor segmaps have problems with shear bias in the direction of neighbors in blended galaxies. From Jarvis 2016: tests revealed significant biases from the masking procedure we had been using, involving just the SEXTRACTOR segmentation maps. When a galaxy had a bright neighbor on the same postage stamp, light from the neighbor that was just outside the segmentation map was being included as part of the fit, thus significantly biasing the inferred shapes in the direction of the neighbor.

how to implement uberseg (again from Jarvis 2016)

1. start with the SEXTRACTOR segmentation map from the coadd image,
2. map it on to the corresponding pixels of the single-epoch images.
3. set pixels in the weight map to zero if they were either associated with other objects in the segmentation map or were closer to any other object than to the object of interest.

Start by tracing where the SExtractor coadd segmentation map and single-epoch weight maps are handled. Implement the requested mapping and masking rules from the issue, including zeroing pixels assigned to other objects or closer to another object, then verify the resulting masks on blended galaxies against the expected shear-bias behavior.