Add pystencils-autodiff

This adds pystencils_autodiff (https://pypi.org/project/pystencils-autodiff/0.1.3/) to pystencils.

After installing the extension, you can access all its classes in the submodule pystenicls.autodiff. If it's not installed but you try to import it you get an error with installation instructions.

The internal code of pystencils_autodiff is still very ugly. I hope I can clean it up in the next days/weeks.