GP subclasses that implement appropriate defaults

[edaub]

I was talking with Louise about stochastic emulators, and one idea that came up was how subclasses might be used to suggest sensible defaults for different types of UQ problems. For instance, if I'm doing a stochastic simulation, then I use a StochasticGP (probably a bad name but hopefully you get the idea) that has defaults like kernel and nugget handling that make sense for this type of problem.