title: xarray firing a Future Warning
body: <p>Just upgraded to python3.8, I have updated xarray to v.0.16, but now I am always getting this warning:</p>
  <pre><code>/usr/local/lib/python3.8/dist-packages/xarray/core/common.py:1123: FutureWarning: 'base' in .resample() and in Grouper() is deprecated.
  The new arguments that you should use are 'offset' or 'origin'.
  
  &gt;&gt;&gt; df.resample(freq=&quot;3s&quot;, base=2)
  
  becomes:
  
  &gt;&gt;&gt; df.resample(freq=&quot;3s&quot;, offset=&quot;2s&quot;)
  
    grouper = pd.Grouper(
  </code></pre>
  <p>The only point in my script in which I am using .resample is this:</p>
  <pre><code>mydata = xr.open_dataset(ncfile).resample(time='3H').reduce(np.mean)
  </code></pre>
  <p>but I don't know how to change it to avoid the warning.</p>