Study overscan subtraction problems

Study the overscan subtraction of CCD raw data as part of the preprocessing. e.g. 20190621/00016474 z0 has a clear step between amplifiers that might be an algorithmic robustness problem:
and amps C and D of 20190306/00001076 z2 have row-to-row variations that look like we need row-by-row subtraction (or maybe better bias image):

Track these down and see if any algorithmic updates are needed, or otherwise feedback to the hardware team if it appears to be a raw data problem.