resample DeprecationWarning only on 1-D arrays?

[raybellwaves]

Code Sample, a copy-pastable example if possible

da = xr.DataArray(np.array([1,2,3,4], dtype=np.float).reshape(2,2),
...               coords=[pd.date_range('1/1/2000', '1/2/2000', freq='D'),
...                       np.linspace(0,1,num=2)],
...               dims=['time', 'latitude'])

da.resample(freq='M', dim='time', how='mean')
#/Users/Ray/anaconda/envs/rot-eof-dev-env/bin/ipython:1: DeprecationWarning: 
#.resample() has been modified to defer calculations. Instead of passing 'dim' and 'how="mean", #instead consider using .resample(time="M").mean() 
#  #!/Users/Ray/anaconda/envs/rot-eof-dev-env/bin/python
#Out[66]: 
#<xarray.DataArray (time: 1, latitude: 2)>
#array([[2., 3.]])
#Coordinates:
#  * time      (time) datetime64[ns] 2000-01-31
#  * latitude  (latitude) float64 0.0 1.0

da.resample(time="M").mean()
#<xarray.DataArray (time: 1)>
#array([2.5])
#Coordinates:
#  * time     (time) datetime64[ns] 2000-01-31

Problem description

The DeprecationWarning example seems to only work for 1d arrays as it doesn't average along any dimension.

A quick fix could be to show the warning only if the DataArray/Dataset is 1D.

A more thorough fix could be to wrap .resample(time="M").mean() as .resample(freq='M', dim='time', how='mean')???

Expected Output

Same as da.resample(freq='M', dim='time', how='mean')

Output of xr.show_versions()

xr.show_versions() # Not sure about the h5py FutureWarning? /Users/Ray/anaconda/envs/rot-eof-dev-env/lib/python3.6/site-packages/h5py/__init__.py:36: FutureWarning: Conversion of the second argument of issubdtype from `float` to `np.floating` is deprecated. In future, it will be treated as `np.float64 == np.dtype(float).type`. from ._conv import register_converters as _register_converters

INSTALLED VERSIONS

commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Darwin
OS-release: 17.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

xarray: 0.10.3
pandas: 0.22.0
numpy: 1.14.2
scipy: 1.0.1
netCDF4: 1.3.1
h5netcdf: 0.5.1
h5py: 2.7.1
Nio: None
zarr: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.17.2
distributed: 1.21.6
matplotlib: 2.2.2
cartopy: 0.16.0
seaborn: None
setuptools: 39.0.1
pip: 9.0.3
conda: None
pytest: None
IPython: 6.3.1
sphinx: None

[shoyer]

I think the correct alternative is da.resample(time="M").mean('time'). We should probably update the warning message.