Add support for Zarr data I/O format

[forman]

xarray 0.10 introduced two new methods, xr.open_zarr() and Dataset.to_zarr.

After a few first tests, I am very enthusiastic about the Zarr, a new data format optimized for distributed and concurrent array I/O. It seems to offer much better I/O performance over NetCDF4, which maybe due to single-threaded HDF5 decompression in Python (not checked).

As it seems to be a 1:1 representation of the NetCDF4 / HDF5 data model, Cate could use it for very efficient workspaces persistence or users could use it for intermediate computation results.

The good news is, that Cate doesn't require any extra dependencies as the zarr package is already a dependency of xarray 0.10.

[JanisGailis]

After quickly googling around and reading about Zarr, it really looks quite impressive.

[forman]

And it is lightning fast. Just ingested a Zarr data cube with dims=(time=250, lat=1000, lon=2000) (for another project) and when I time-travel through it, layers are displayed immediately.

[papesci]

it looks an impressive persistence support. It would be a good idea to include it enabling the user to save processed data.

[forman]

I'll merge my branch, so we can play with it.