Black box external likelihood notebook

[ccaprani]

Using a “black box” likelihood function (numpy) Notebook url

Issue description

The updates done as part of #28 changed quite a few of the functions so that the text no longer matches the code. @rayhagimoto has left a very useful comment on this. There is no longer a finite difference calculation, in spite of what the text says.

Proposed solution

The text needs updating to be consistent with the code changes. And consistent with the original notebook on which this was based, there should be a finite-difference example added.

If I get some time I'll look at this, but in the meantime I'm noting this as an issue.