Support for nested dictionaries in atttributes for Zarr

[eddienko]

Similar to #2868, but for Zarr. In this case Zarr should support saving nested dictionaries.

import xarray as xr
ds = xr.Dataset()
ds.attrs['foo'] = {'bar': 1}
ds.to_zarr('foo.zarr', mode='w')

Error message:

TypeError                                 Traceback (most recent call last)
<ipython-input-11-4e7ad4f0a599> in <module>
----> 1 ds.to_zarr('ll.zarr', mode='w')

/srv/conda/envs/notebook/lib/python3.7/site-packages/xarray/core/dataset.py in to_zarr(self, store, mode, synchronizer, group, encoding, compute, consolidated, append_dim)
   1614             compute=compute,
   1615             consolidated=consolidated,
-> 1616             append_dim=append_dim,
   1617         )
   1618 

/srv/conda/envs/notebook/lib/python3.7/site-packages/xarray/backends/api.py in to_zarr(dataset, store, mode, synchronizer, group, encoding, compute, consolidated, append_dim)
   1299     # validate Dataset keys, DataArray names, and attr keys/values
   1300     _validate_dataset_names(dataset)
-> 1301     _validate_attrs(dataset)
   1302 
   1303     if mode == "a":

/srv/conda/envs/notebook/lib/python3.7/site-packages/xarray/backends/api.py in _validate_attrs(dataset)
    209     # Check attrs on the dataset itself
    210     for k, v in dataset.attrs.items():
--> 211         check_attr(k, v)
    212 
    213     # Check attrs on each variable within the dataset

/srv/conda/envs/notebook/lib/python3.7/site-packages/xarray/backends/api.py in check_attr(name, value)
    204                 "a string, an ndarray or a list/tuple of "
    205                 "numbers/strings for serialization to netCDF "
--> 206                 "files".format(value)
    207             )
    208 

TypeError: Invalid value for attr: {'bar': 1} must be a number, a string, an ndarray or a list/tuple of numbers/strings for serialization to netCDF files

Additionally the error message mentions netCDF -- even if I am writing a Zarr file.

Workaround is to save as a list, i.e.

ds.attrs['foo'] = [{'bar': 1}]

[dcherian]

Thanks for opening an issue @eddienko.

It looks like we'll need to validate attrs based on whether backend="zarr" or backend="netCDF4"

[eddienko]

Had a go at this -- first PR here, so please review carefully.