Add annealed importance sampling

[ben18785]

Key paper: https://arxiv.org/abs/physics/9803008

Possibly the most widely used approach to calculating the marginal likelihood.

[ben18785]

We may want a whole new class of methods (alongside optimisation and sampling algorithms), which calculate the marginal likelihood of models. Marginal likelihood calculation is really important in physics, and if we were to become the de facto standard on this, citations would follow...