Refactor GaussianProcesses

[odunbar]

Issue

Though we have modularity/extensibility of the current GaussianProcess framework. It currently depends to heavily on the similarity of how GP packages work and the objects they create.

Currently we create a GaussianProcess{<:GPPackage} object and dispatch off the package. And this gets quite messy as, for example we end up with many Union-types of the fields inside this object.

Possible solution

I think a better solution is to keep the GPPackage types and the GaussianProcess could now become an abstract type. Then we can create (bad names->) SKLJLGP, GPJLGP, AGPJLGP, ... <: GaussianProcess, that is, different objects for the different packages.

These can contain a variety of different fields and options, and will have typed fields that avoids needing unions between different packages