in genDefectListTask.py use afwDetect to find bad pixels

please use afwDetect to find the defects, then lsst::afw::detection::footprintToBBoxList to generate the defects (or simply iterate over the Footprints if you're unfamiliar with that function). Actually, you can use ip.isr.defectListFromFootprintList

You should almost never be iterating over images pixel-by-pixel at the python level. If any parts of drp_stella are doing so, please open issues to fix them.