Code Sample, a copy-pastable example if possible

A "Minimal, Complete and Verifiable Example" will make it much easier for maintainers to help you: http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports

```python

Your code here

def xr_reshape(A, dim, newdims, coords): """ Reshape DataArray A to convert its dimension dim into sub-dimensions given by newdims and the corresponding coords. Example: Ar = xr_reshape(A, 'time', ['year', 'month'], [(2017, 2018), np.arange(12)]) """

# Create a pandas MultiIndex from these labels
ind = pd.MultiIndex.from_product(coords, names=newdims)

# Replace the time index in the DataArray by this new index,
A1 = A.copy()

A1.coords[dim] = ind

# Convert multiindex to individual dims using DataArray.unstack().
# This changes dimension order! The new dimensions are at the end.
A1 = A1.unstack(dim)

# Permute to restore dimensions
i = A.dims.index(dim)
dims = list(A1.dims)

for d in newdims[::-1]:
    dims.insert(i, d)

for d in newdims:
    _ = dims.pop(-1)


return A1.transpose(*dims)

```

Problem description

[this should explain why the current behavior is a problem and why the expected output is a better solution.]

It would be great to have the above function as a DataArray's method.

Expected Output

A reshaped DataArray. In the example in the function comment it would correspond to an array like

In[1] Ar.dims Out[1]: ('year', 'month', 'lat', 'lon')

Output of xr.show_versions()

Hi guys!

Thanks for the great job you are doing!

Here is a possible bug in the Dataset.combine_first() method (I'm using xarray 0.9.2 with Python 3.5.2).

The Dataset/DataArray coordinates are correctly treated if specified without commas, e.g. (as seen with ncdump): MyVar:coordinates = "alt lat lon" ;. In this case lon, lat, and alt are recognized as the Dataset's (and its DataArrays') .coords.

However, the Dataset.combine_first()method only treats coordinates correctly if specified with commas, i.e., MyVar:coordinates = "alt, lat, lon".