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.