Expectation examples

[wiseodd]

Example of how to leverage the newly added functional_samples method #243.

I also added the abstract interface for samples and functional_samples to BaseLaplace.

[runame]

Besides my comments, LGTM.

[aleximmer]

LGTM, thanks for adding the example. Some comment on a higher level but not necessary to add: it would be cool to plot the EI function and then compute the gradient at some point and add that to the plot as well to visualize the example mot intuitively.