return standard error of estimates

[jeremymanning]

Feature request from Mike Kahana: return standard error of estimates (in addition to the means)

We could potentially swap in this model for Model.predict and Model.update: http://scikit-learn.org/stable/modules/gaussian_process.html

[jeremymanning]

this also looks good: http://george.readthedocs.io/

[jeremymanning]

Another option: https://gpytorch.readthedocs.io/en/latest/index.html