Expectation Propagation

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Basic tasks:

• EP marginal likelihood approximation (RW5) & tests for optimising hyperparameters
• more efficient prediction (RW3) instead of piggybacking on SVGP
• rank-1 updates of q(f) after each site update in the inner loop
• error checking - e.g. div_dist might not be valid!
• custom handling of canonical parametrisation to allow initialisation at precision=0?

Extended tasks:

• sparse EP
• PowerEP