title: get mean of netcdf file using xarray
body: <p>I have opened a netcdf file in python using xarray, and the dataset summary looks like this.</p>
  <pre><code>Dimensions:    (latitude: 721, longitude: 1440, time: 41)
  Coordinates:
    * longitude  (longitude) float32 0.0 0.25 0.5 0.75 ... 359.25 359.5 359.75
    * latitude   (latitude) float32 90.0 89.75 89.5 89.25 ... -89.5 -89.75 -90.0
      expver     int32 1
    * time       (time) datetime64[ns] 1979-01-01 1980-01-01 ... 2019-01-01
  Data variables:
      z          (time, latitude, longitude) float32 50517.914 ... 49769.473
  Attributes:
      Conventions:  CF-1.6
      history:      2020-03-02 12:47:40 GMT by grib_to_netcdf-2.16.0: /opt/ecmw...
  </code></pre>
  <p>I want to get the mean of the values of z along the latitude and longitude dimensions.</p>
  <p>I've tried to use this code:</p>
  <p><code>df.mean(axis = 0)</code></p>
  <p>But it's removing the time coordinate, and returning me something like this.</p>
  <pre><code>Dimensions:  (latitude: 721, longitude: 1440)
  Coordinates:
      expver   int32 1
  Dimensions without coordinates: latitude, longitude
  Data variables:
      z        (latitude, longitude) float32 49742.03 49742.03 ... 50306.242
  </code></pre>
  <p>Am I doing something wrong here. Please help me with this.</p>