Refactor GaussianProcesses

## 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