Addition of a task_0, introduction to Monte Carlo

[Allentro]

Here's my first attempt at a task 0. It would be great to get feedback on how to improve my initial sketch up. #172

[shimwell]

I'm wondering if this might fit in better between tasks 3 and 4.
Partly because there are also deterministic methods for neutronics that we don't cover here and MC it is not prerequisite for tasks 1 and 2 (nuclear data and csg geometry).