[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/cpu/kernelcreation.py

@@ -127,7 +127,10 @@ def create_indexed_kernel(assignments: Union[AssignmentCollection, NodeCollectio
 
     non_index_fields = [f for f in all_fields if f not in index_fields]
     spatial_coordinates = {f.spatial_dimensions for f in non_index_fields}
-    assert len(spatial_coordinates) == 1, "Non-index fields do not have the same number of spatial coordinates"
+    assert len(spatial_coordinates) == 1, f"Non-index fields do not have the same number of spatial coordinates " \
+                                          f"Non index fields are {non_index_fields}, spatial coordinates are " \
+                                          f"{spatial_coordinates} and len(spatial coordiantes) is " \
+                                          f"{len(spatial_coordinates)}"
     spatial_coordinates = list(spatial_coordinates)[0]
 
     def get_coordinate_symbol_assignment(name):