Allow results to be returned as an ArviZ InferenceData object

ArviZ provides a nice way to store and visualise samples from a variety of inference packages. It would be good to provide a method of the Results class that converts the results to an Arviz InferenceData object.

A more controversial suggestion would be to have the bilby Results class actually itself be a child of the ArviZ InferenceData class, but that would require a more considerable rewrite.