Alignment: fixed logic error

Signed-off-by: Nicola VIGANÒ <nicola.vigano@esrf.fr>

nabu/preproc/alignment.py

@@ -584,7 +584,7 @@ class DetectorTranslationAlongBeam(AlignmentBase):
         if equispaced_increments:
             img_pos_increments = img_pos[1:] - img_pos[0]
         else:
-            img_pos_increments = - np.diff(img_pos)
+            img_pos_increments = np.diff(img_pos)
 
         # Polynomial.fit is supposed to be more numerically stable than polyfit
         # (according to numpy)