feat: Monte Carlo integrator.

[daniel-dodd]

Add Monte Carlo integrator to compute the variational expectation / expected log likelihood: $$\mathbb{E}_{q(\mathbf{f})} \log p(y|\mathbf{f}) \approx \frac{1}{N} \sum_i \log p(y|\mathbf{f}^{(i)}), \qquad \mathbf{f}^{(i)} \sim q(\mathbf{f}). $$

class MonteCarloIntegrator(AbstractIntegrator): 
   ...

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