GP Sampling

[Crown421]

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Summary This PR is a (at this stage) draft PR to allow sampling of GP, including sampling via Cholesky, via Random Fourier Features, and pathwise/ decoupled sampling.

Proposed changes The main interface revolves around a new GPSampler struct, as a subtype of Random.Sampler, which takes a GP and a "method". One can then draw from this sampler using rand`.

However, there are still issues to this implementation

• GP must have a kernel (when using RFF)
• Not yet working with sparse approximations
• Pathwise functionality was written to (theoretically) enable using Cholesky samples for the prior, and RFF for the posterior, but in practice one would never want to do that.
  • It might not be worth to keep this use case around
• Multi-Output probably not working yet (and not tested)

What alternatives have you considered? Making a separate package, and/or (due to the complexity) just keeping it good enough for my needs.

Breaking changes Should not be breaking (at this point).