Make sure that ArviZ.jl is compatible with arviz v0.8

[sethaxen]

The changelog is here: https://github.com/arviz-devs/arviz/blob/master/CHANGELOG.md#v080-2020-may-23

In addition to making sure tests pass, also make sure that new features in arviz that have ArviZ.jl analogs also work here.

[sethaxen]

Going through the changelog, these appear to be things we need to look at:

• Allow xarray.Dataarray input for plots.(#1120)
• Revamped the hpd function to make it work with mutidimensional arrays, InferenceData and xarray objects (#1117)
• Add out-of-sample groups (predictions and predictions_constant_data) to from_dict (#1125)
• Add out-of-sample groups (predictions and predictions_constant_data) and constant_data group to pyro and numpyro translation (#1090, #1125)
• Integrate jointplot into pairplot, add point-estimate and overlay of plot kinds (#1079)
• Add warmup groups to InferenceData objects, initial support for PyStan (#1126) and PyMC3 (#1171)
• hpd function deprecated in favor of hdi. credible_interval argument replaced by hdi_probthroughout with exception of plot_loo_pit (#1176)
• plot_hpd function deprecated in favor of plot_hdi. (#1190)

[sethaxen]

At this point, the only features we appear to be missing are support for saving warmup for MCMCChains .Chains and (maybe) NamedTuple inputs. But those aren't yet supported for all from_XXX methods in ArviZ, so I think this issue is indeed closed.