Initial port commit

[willtebbutt]

This ports a lot of initial work over from Stheno.jl. There's still some stuff to do, but intended use should be clear from the tests.

Left to do:

• Implement approximate posterior type to handle VFE / DTC appropriately.
• Reintroduce doctests + fix any errors.
• Specify the AbstractGP interface in abstract_gp.jl + document.
• Write a readme
• Write some initial docs

edit: the readme will suffice for this PR.

edit2: added standardised testing for the AbstractGP interface to make it easy to check that it's been implemented fully and self-consistently.

[willtebbutt]

Note that tests are currently failing because I'm waiting for this PR to be merged into KernelFunctions, and at least a new patch version to be released.

edit: I actually just need a patch version bump.

[sharanry]

Had a few suggestions and doubts. Thanks!

[codecov-io]

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[willtebbutt]

@sharanry reading for another round of review. Let me know what you think.

[willtebbutt]

Thanks for the extensive reviewing @sharanry . Will merge once tests pass.