Use NetCDF objects to store posteriors

[fonnesbeck]

Rather than storing zipped JSON files, it would be nice to have the posteriors stored in a richer file format. Given the ubiquity of the ArviZ library and its compatibility with virtually all PPLs, would you ever consider switching over to storing a NetCDF (or xarray, or parquet) export of the posteriors? This would obviate the need to store parts of the output in different subdirectories, since the draws, diagnostics and other info could all be stored together.