[Flatfield] Histogram settings in attributes.json are ignored on restart

[igorpisarev]

If the flatfield estimation task is restarted after the histograms have been written out, the code ignores the histogram settings stored in the attributes.json file and scans through the entire stack to recompute them. Reading the stored settings will make it faster and will eliminate the possibility where different values are used for generating the histograms and later for estimating the flatfield.