Support new arviz v0.10.0 features

[sethaxen]

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Relevant entries from the changelog: New plots:

• Added plot_separation (1359)

Methods for InferenceData:

• Added to_dict method for InferenceData object (1223)
• Extended methods from xr.Dataset to InferenceData (1254)
• Add extend and add_groups to InferenceData (1300 and 1386)
• Added __iter__ method (.items) for InferenceData (1356)

New examples:

• Add more plot_parallel examples (1380)

These can already be accessed with the forwarded dot syntax (i.e. idata.to_dict() already works), but we need to figure out if any of these should also be overloads of Base functions.