Is it possible to append_dim to netcdf stores

[hmaarrfk]

Is your feature request related to a problem? Please describe.
Feature request: It seems that it should be possible to append to netcdf4 stores along the unlimited dimensions. Is there an example of this?

Describe the solution you'd like
I would like the following code to be valid:

from xarray.tests.test_dataset import create_append_test_data
ds, ds_to_append, ds_with_new_var = create_append_test_data()

filename = 'test_dataset.nc'
# Choose any one of
# engine : {'netcdf4', 'scipy', 'h5netcdf'}
engine = 'netcdf4'
ds.to_netcdf(filename, mode='w', unlimited_dims=['time'], engine=engine)
ds_to_append.to_netcdf(filename, mode='a', unlimited_dims=['time'], engine=engine)

Describe alternatives you've considered
I guess you could use zarr, but the fact that it creates multiple files is a problem.

Additional context
xarray version: 0.16.0

[dopplershift]

Duplicate of #1672?

[dcherian]

Thanks! Closing as duplicate of #1672