Make COR conversion from rel to abs consistent with Nabu

The output of the COR estimators is a relative COR position, i.e. a position relative to the center of the detector. Then, this relative position is converted to an absolute one, i.e. with respect to the left side of the detector. The conversion is done with:

abs = rel + (detector_width - 1)// 2.

This corresponds to taking the origin of the detector at the center of the pixel of the first column. In nabu, the CompositeCORFinder does the same conversion, contrary to CORFinder and SinoCORFinder classes (see Nabu #486).