[BugFix] Fix indexing with ghostlayers

The Block indexing has bug when created with an iteration slice and ghost layers. With !341 (merged) The Block indexing supports slices more naturally by limiting the iteration space to the sliced size. Thus the counter index is multiplied by the step size. This was done also for the offset of the ghostlayers which is wrong.

This MR fixes the problem

pystencils/gpu/indexing.py

@@ -398,7 +398,8 @@ def _loop_ctr_assignments(loop_counter_symbols, coordinates, iteration_space):
     loop_ctr_assignments = []
     for loop_counter, coordinate, iter_slice in zip(loop_counter_symbols, coordinates, iteration_space):
         if isinstance(iter_slice, slice) and iter_slice.step > 1:
-            loop_ctr_assignments.append(SympyAssignment(loop_counter, coordinate * iter_slice.step - iter_slice.start))
+            offset = (iter_slice.step * iter_slice.start) - iter_slice.start
+            loop_ctr_assignments.append(SympyAssignment(loop_counter, coordinate * iter_slice.step - offset))
         elif iter_slice.start == iter_slice.stop:
             loop_ctr_assignments.append(SympyAssignment(loop_counter, 0))
         else: