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/llvm/kernelcreation.py

@@ -26,4 +26,5 @@ def create_kernel(assignments, function_name="kernel", type_info=None, split_gro
     from pystencils.cpu import create_kernel
     code = create_kernel(assignments, function_name, type_info, split_groups, iteration_slice, ghost_layers)
     code = insert_casts(code)
+    code.backend = 'llvm'
     return code