Need to update ImageMapReduce to return 2 exposures (diffim and matched template, used for dipole fitting). This is currently not implemented (for spatially Zogy) but I added a check in imageDifference.py so that if it doesn't exist in the returned Struct, then it is not used.

Zogy has been integrated into `imageDifference.py` in this ticket. The means of integration is not ideal, I admit, but this is due to the current organization of the `imageDifference.py` `ImageDifferenceTask.run` method itself. I decided not to refactor that method here, as that is subject to another ticket (DM-5295and DM-3704). After this ticket is merged, I will prioritize DM-5295 and DM-3704 to facilitate further improvements to the image differencing task.

Both the constant-across-the-image and the spatially-varying versions of Zogy are available. By default the constant one is enabled (config `doSpatiallyVarying=False`), but after further testing this default could be changed.

For some reason, measurement is very slow when using the spatiallyVarying (ALdecorrelation and Zogy) options.
Possibly due to use of the CoaddPsf for those resulting diffims. To be evaluated in DM-10806.

Another issue identified: Zogy seems to lead to slightly too many positive (~20%) and way too many (~2x) negative detections. Need to investigate this. It is partly due to the artifacts around bright stars (see comment below) but also seen in vicinity of image edges.

Zogy diffim's have strange "fingerprint"-like artifacts around bright (but unsaturated) stars. Possibly a result of the sinc interpolation used for warping. This needs to be investigated.