[Fix] GPU Buffer with iteration slices

GPU buffers did not work in combination with iteration slices before. This is resolved here

pystencils/gpucuda/kernelcreation.py

@@ -44,7 +44,9 @@ def create_cuda_kernel(assignments: Union[AssignmentCollection, NodeCollection],
         field_accesses = {e for e in field_accesses if not e.is_absolute_access}
         num_buffer_accesses += sum(1 for access in eq.atoms(Field.Access) if FieldType.is_buffer(access.field))
 
+    # common shape and field to from the iteration space
     common_shape = get_common_shape(fields_without_buffers)
+    common_field = list(sorted(fields_without_buffers, key=lambda e: str(e)))[0]
 
     if iteration_slice is None:
         # determine iteration slice from ghost layers
@@ -62,7 +64,7 @@ def create_cuda_kernel(assignments: Union[AssignmentCollection, NodeCollection],
                 iteration_slice.append(slice(ghost_layers[i][0],
                                              -ghost_layers[i][1] if ghost_layers[i][1] > 0 else None))
 
-    indexing = indexing_creator(field=list(fields_without_buffers)[0], iteration_slice=iteration_slice)
+    indexing = indexing_creator(field=common_field, iteration_slice=iteration_slice)
     coord_mapping = indexing.coordinates
 
     cell_idx_assignments = [SympyAssignment(LoopOverCoordinate.get_loop_counter_symbol(i), value)