+ *
+ * When an element is missing, we suppose slices join together
+ * (no overlapping, no interslice gap) but we have no way to check it !
+ * For *Dicom* images, ZSpacing *should be* calculated using
+ * XOrigin, YOrigin, ZOrigin (of the top left image corner)
+ * of 2 consecutive images, and the Orientation
+ * Computing ZSpacing on a single image is not really meaningfull !
+