Gaussian Random Field parameters: Phase 2

[odunbar]

With #252 merged, we have raised some new issues to accompany the GRF tools.

- [ ] Check toml interface
- [ ] https://github.com/CliMA/EnsembleKalmanProcesses.jl/issues/317
- [x] Document GRF interface / parameter distribution
- [ ] Convenient constructor for a GRF with prescribed mean & spatial correlation
- [ ] Compatability with new plot features

[odunbar]

Deal with #300 here too

• [x] Update Base.show for function distributions

[odunbar]

• [ ] As everything is stacked internally, we should provide an API to obtain the evaluation point grid, for easy reshaping (if we are plotting for example).