title: Output int32 time dimension in netCDF using xarray
body: <p>Let's say I have time data that looks like this in an xarray Dataset:</p>
  
  <pre><code>ds = xr.Dataset({'time': pd.date_range('2000-01-01', periods=10)})
  ds.to_netcdf('asdf.nc')
  </code></pre>
  
  <p>xarray's <code>to_netcdf()</code> method outputs the time dimension as int64:</p>
  
  <pre><code>$ ncdump -v time asdf.nc
  netcdf asdf {
  dimensions:
      time = 10 ;
  variables:
      int64 time(time) ;
          time:units = "days since 2000-01-01 00:00:00" ;
          time:calendar = "proleptic_gregorian" ;
  data:
  
   time = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ;
  }
  </code></pre>
  
  <p>Because I'm working with a THREDDS server which does not support int64, I would like for these time data to be int32. Is this possible to do using xarray?</p>