AbstractData check with parent before throwing AttributeError
fcalc.processFiles(background=100) subtracts a constant bg
Added imports to fplot for fplot.hist
Added ROI error checking and reporting

ROI checks for sane values when creating a new ROI, and also
offers better exception reporting for various bad boundaries.

ROI.toRelative() will also now return a new ROI with translated
coordinates, or else reports even more error information.

This is to avoid the problem of silently breaking things when an
ROI is actually out of bounds on an image. Translating it will create
negative bounds. Fail early!