fixes strange bug in cube file format

Author: Marcello Sega

pytim/cube.py

@@ -88,7 +88,7 @@ def write_file(filename,
         else:
             natoms = 0
         grid_size = np.array(grid_size, dtype=int)
-        shift = np.array([1, 1, 1]) * spacing / 2.
+        shift = -np.array([0, 0, 11 ]) * spacing  * Bohr
         #spacing =  spacing*(grid_size+1.)/grid_size
         f.write('CPMD CUBE FILE\n')
         f.write('GENERATED BY PYTIM\n')