Add `pystencils.make_python_function` used for KernelFunction.compile

KernelFunction.compile = None is currently set by the create_kernel function of each respective backend as partial function of <backend>.make_python_function.

The code would be clearer with a unified make_python_function. KernelFunction.compile can then be implemented as a call to this function with the respective backend.

pystencils/cpu/kernelcreation.py

+from typing import List, Union
+
 import sympy as sp
-from functools import partial
-from pystencils.astnodes import SympyAssignment, Block, LoopOverCoordinate, KernelFunction
-from pystencils.transformations import resolve_buffer_accesses, resolve_field_accesses, make_loop_over_domain, \
-    add_types, get_optimal_loop_ordering, parse_base_pointer_info, move_constants_before_loop, \
-    split_inner_loop, get_base_buffer_index, filtered_tree_iteration
-from pystencils.data_types import TypedSymbol, BasicType, StructType, create_type
-from pystencils.field import Field, FieldType
+
 import pystencils.astnodes as ast
-from pystencils.cpu.cpujit import make_python_function
 from pystencils.assignment import Assignment
-from typing import List, Union
+from pystencils.astnodes import (Block, KernelFunction, LoopOverCoordinate,
+                                 SympyAssignment)
+from pystencils.data_types import (BasicType, StructType, TypedSymbol,
+                                   create_type)
+from pystencils.field import Field, FieldType
+from pystencils.transformations import (add_types, filtered_tree_iteration,
+                                        get_base_buffer_index,
+                                        get_optimal_loop_ordering,
+                                        make_loop_over_domain,
+                                        move_constants_before_loop,
+                                        parse_base_pointer_info,
+                                        resolve_buffer_accesses,
+                                        resolve_field_accesses,
+                                        split_inner_loop)
 
 AssignmentOrAstNodeList = List[Union[Assignment, ast.Node]]
 
@@ -83,7 +91,6 @@ def create_kernel(assignments: AssignmentOrAstNodeList, function_name: str = "ke
         resolve_buffer_accesses(ast_node, get_base_buffer_index(ast_node), read_only_fields)
     resolve_field_accesses(ast_node, read_only_fields, field_to_base_pointer_info=base_pointer_info)
     move_constants_before_loop(ast_node)
-    ast_node.compile = partial(make_python_function, ast_node)
     return ast_node
 
 
@@ -148,7 +155,6 @@ def create_indexed_kernel(assignments: AssignmentOrAstNodeList, index_fields, fu
     read_only_fields = set([f.name for f in fields_read - fields_written])
     resolve_field_accesses(ast_node, read_only_fields, field_to_fixed_coordinates=fixed_coordinate_mapping)
     move_constants_before_loop(ast_node)
-    ast_node.compile = partial(make_python_function, ast_node)
     return ast_node