Implementing Zarr I/O

[sethaxen]

As noted in arviz-devs/ArviZ.jl#207 (comment), we should support I/O to/from Zarr files using Zarr.jl.

[sethaxen]

The right way to go here is probably to implement a simple package InferenceDataZarr, similar to InferenceDataNetCDF with just two methods:

• from_zarr
• to_zarr.

This is probably pretty straightforward by patterning the design off of InferenceDataZarr and following the Zarr API, so this is a good first issue for someone who's registered a Julia package before.