Move InferenceData examples to own repo

[sethaxen]

We already moved the data files themselves to their own repo, but it would additionally be useful to have a lightweight repo ArviZExampleData.jl that contains the code for loading these examples and the function load_example_data currently in this repo. This repo would then be reexported here.

ArviZExampleData could then be used as a test dependency for InferenceObjects.jl and other repos that may want to test against typical InferenceData objects.

An alternative name could be InferenceObjectsExamples.jl or InferenceDataExamples.jl.

[sethaxen]

Examples now live at https://github.com/arviz-devs/ArviZExampleData.jl, and we can expose the loading functions here.