Moment matching rules for Gaussian+{Delta,Gaussian} etc.

[fritzo]

@lawmurray pointed out that, while the moment_matching interpretation allows Gaussians to be closed under marginalization of the form .reduce(ops.logaddexp, misc_discrete_vars), it currently does not handle binary mixtures of the form Gaussian + GaussianMixture or even Gaussian + Gaussian. We could conceivably add new approximation rules, uncontroversially for Gaussian, but also possibly for Gaussian summarizations of mixtures of Deltas.

Possible rules to add

• [ ] Gaussian + Gaussian => Gaussian
• [ ] Gaussian + Delta => Gaussian
• [ ] Delta + Delta => Gaussian
• [ ] Reduce(LogaddexpOp, Delta, -) => Gaussian

[fritzo]

cc @martinjankowiak

[eb8680]

These will also be necessary for #90