title: Resampling xarray DataSet using different methods
body: <p>I need to resample a <code>DataSet</code> using <code>mean()</code> for one, <code>sum()</code> for another data variable. I am using this somewhat cumbersome method:</p>
  
  <pre><code>import xarray as xr
  import numpy as np
  
  airtemps = xr.tutorial.load_dataset('air_temperature')
  airtemps = airtemps.sel(time=slice('2013-01-01', '2013-12-31'))
  airtemps['air'] = airtemps.air - 273.15
  airtemps['data'] = (('time', 'lat', 'lon'), np.random.random(airtemps.air.shape))
  
  dataSummed = airtemps.data.resample('M', dim='time', how='sum')
  airtemps_month = airtemps.resample('M', dim='time', how='mean')
  airtemps_month['data_summed'] = dataSummed
  </code></pre>
  
  <p>Can/should this be done differently?</p>