Restructure demos into more functions where appropriate

Most demos (C++ and Python) have a flat structure. This is fine for simple problems, but I see time and time again that users model their more extensive solvers on the demos and create single function (C++) / flat script (Python) demos that can be many hundreds or thousands of lines.

It would be good to review the structure of demos and refactor with more functions where appropriate. Hopefully users will then model their own codes on the more structured demos.