add more tutorials

[LegrandNico]

The following tutorials need to be expanded and rewritten:

• [x] Multilevel embedding of Hierarchical Gaussian Filters
• [x] Parameters recovery, prior predictive and posterior predictive sampling
• [ ] The cardiac filtering HGF could include an example of seasonality decomposition through value coupling

We need new tutorials on the following topics:

• [ ] Binary HGF with finite precision
• [x] The multivariate normal HGF (once #144 completed)