Add support for Beta distribution in Generalised Bayesian Filters nodes

[LegrandNico]

Generalised Bayesian filtering is described in this paper.

We need to implement a node that uses:

• [ ] A fixed learning rate
• [ ] A dynamic learning rate (i.e. using continuous nodes to filter sufficient statistics)

The first steps will be to:

• [ ] Add a Beta class in the math.py file to convert between the different components of a Beta distribution.

See this page for the details on the parameterisation of a beta distribution, and this page can also be useful.