See Requirements for overhauled calibration task? for a map of what processCcd.py is likely to do after refactoring CalibrateTask. @jbosch’s suggestion near the end seems likely to be adopted.
Also, I am keeping https://jira.lsstcorp.org/browse/DM-4692 up to date with what I am actually implementing. Comments are welcome, but preferably on this forum using Requirements for overhauled calibration task? or a discussion linked from there.