Wrong slice spacing for DICOM Seg

[ivmartel]

DICOM segs get loaded with a wrong pixel spacing. This triggers the 'Test distance is increasing when adding intermediate pos pats' error.
The code uses first the 'Slice thickness' tag (0018,0050) and then the 'Spacing between slices' tag (0018,0088). This logic should be reversed since the spacing includes the thickness and some possible gap.