title: xarray: replace array values corresponding to particular dates in datetime
body: <p>I have an example array of zeros: </p>
  
  <pre><code>time = np.arange('2000', '2005', dtype='datetime64[D]')
  test_array = xr.DataArray(np.zeros(len(time)), coords={'time': time}, dims=['time'])
  </code></pre>
  
  <p>Now if I have some data e.g. <code>test_data = np.ones(365)</code> that I want to put into the array corresponding to year <code>2001</code> (which has 365 days) how do I go about doing this? </p>
  
  <p>I want to do something like: <code>test_array[test_array.where(time='2001')] = test_data</code> but <code>.where()</code> here doesn't work. </p>