make multidimensional slice into a tuple, rather than a list

In the future numpy will not allow multidimensional slices that
are specified as sequences other than tuples.

Closes #210.

kwant/plotter.py

@@ -1943,7 +1943,7 @@ def interpolate_current(syst, current, relwidth=None, abswidth=None, n=9):
             # Zero volume: nothing to do.
             continue
 
-        field_slice = [slice(*slices[i, d]) for d in range(dim)]
+        field_slice = tuple([slice(*slices[i, d]) for d in range(dim)])
 
         # Coordinates of the grid points that are within range of the current
         # hopping.