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/gpucuda/kernelcreation.py

+from pystencils import Field, FieldType
 from pystencils.astnodes import Block, KernelFunction, LoopOverCoordinate, SympyAssignment
 from pystencils.data_types import BasicType, StructType, TypedSymbol
-from pystencils.field import Field, FieldType
 from pystencils.gpucuda.cudajit import make_python_function
 from pystencils.gpucuda.indexing import BlockIndexing
 from pystencils.transformations import (