inconsistent behavior in stack/unstack along one dimension #1431
Description
I am using Ubuntu 16, python 3.6, and xarray 0.9.1
A DataArray can be stacked along one dimension, but when unstack is called a ValueError is raised. It seems that either unstack should work, or calling stack should also raise a ValueError.
import xarray as xr
dims = ['a', 'b']
coords = {'a': range(2), 'b':range(2)}
values = [[0, 0], [0, 0]]
dar = xr.DataArray(values, coords, dims)
stacked = dar.stack(z=('a',)) # this works
unstack = stacked.unstack('z') # this raises ValueError