[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

-from typing import Union
-
 import sympy as sp
 
 import pystencils.astnodes as ast
-from pystencils.simp.assignment_collection import AssignmentCollection
 from pystencils.config import CreateKernelConfig
 from pystencils.enums import Target, Backend
 from pystencils.astnodes import Block, KernelFunction, LoopOverCoordinate, SympyAssignment
@@ -18,7 +15,7 @@ from pystencils.transformations import (
     resolve_field_accesses, split_inner_loop)
 
 
-def create_kernel(assignments: Union[NodeCollection],
+def create_kernel(assignments: NodeCollection,
                   config: CreateKernelConfig) -> KernelFunction:
     """Creates an abstract syntax tree for a kernel function, by taking a list of update rules.
 
@@ -94,7 +91,7 @@ def create_kernel(assignments: Union[NodeCollection],
     return ast_node
 
 
-def create_indexed_kernel(assignments: Union[AssignmentCollection, NodeCollection],
+def create_indexed_kernel(assignments: NodeCollection,
                           config: CreateKernelConfig) -> KernelFunction:
     """
     Similar to :func:`create_kernel`, but here not all cells of a field are updated but only cells with
@@ -115,21 +112,24 @@ def create_indexed_kernel(assignments: Union[AssignmentCollection, NodeCollectio
     fields_written = assignments.bound_fields
     fields_read = assignments.rhs_fields
 
+    all_fields = fields_read.union(fields_written)
+    # extract the index fields based on the name. The original index field might have been modified
+    index_fields = [idx_field for idx_field in index_fields if idx_field.name in [f.name for f in all_fields]]
+    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, 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}"
+    spatial_coordinates = list(spatial_coordinates)[0]
+
     assignments = assignments.all_assignments
     assignments = add_types(assignments, config)
 
-    all_fields = fields_read.union(fields_written)
-
     for index_field in index_fields:
         index_field.field_type = FieldType.INDEXED
         assert FieldType.is_indexed(index_field)
         assert index_field.spatial_dimensions == 1, "Index fields have to be 1D"
 
-    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"
-    spatial_coordinates = list(spatial_coordinates)[0]
-
     def get_coordinate_symbol_assignment(name):
         for idx_field in index_fields:
             assert isinstance(idx_field.dtype, StructType), "Index fields have to have a struct data type"