Have "unstack" return a boolean mask? #3518

arr = xr.DataArray(np.arange(6).reshape(2, 3),
                  coords=[('x', ['a', 'b']), ('y', [0, 1, 2])])
arr
stacked = arr.stack(z=('x', 'y'))
stacked[:4].unstack().dtype

>>> arr = xr.DataArray(np.arange(6).reshape(2, 3),
...                  coords=[('x', ['a', 'b']), ('y', [0, 1, 2])])
>>> arr
<xarray.DataArray (x: 2, y: 3)>
array([[0, 1, 2],
       [3, 4, 5]])
Coordinates:
  * x        (x) <U1 'a' 'b'
  * y        (y) int64 0 1 2
>>> stacked = arr.stack(z=('x', 'y'))
>>> stacked[:4].unstack().dtype
dtype('float64')