mccd focal-plane plots: fixed coordinate flips for psfex

[martinkilbinger]

Summary

With the psfex PSF model the CCD MegaCam coordinate flips were not accounted for. This was correct in the old
MeanShapes.py stand-alone script. Now again fixed.

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[tobias-liaudat]

The changes look good to me, but I have not tested it with data.

The github actions seem to fail in the installation of the pipeline, so it's unrelated to the proposed changes.