fluxCalQA: normalize pfsMerged.flux and anonymize plot

pfsMerged.flux is not divided by exptime and thus the median of (imag_pfsMerged - imag_PS1) varies with exposure time. This has to be fixed. Also, for open-use programs, we want to anonymize the plot (e.g., mask coordinates, etc) in some occasions. Implement the anonymization option.

I know it is not a good manner to implement two features with a single ticket. But, I want to make a quick update for the on-going run and I hope people are not too upset...