Multilevels embedding for the MAB HGF

[LegrandNico]

We need to polish the MAB example a bit and add multilevel solutions:

• [ ] Use a different response function based on the derivative of the continuous node at the 2nd level to account for the increasing uncertainty for unobserved bandits.
• [ ] Add a multilevel example (as far as I can see, once #170 is merged no major technical roadblocks are on the way).