title: Iterating over xarray. DataArray first dimension and its coordinates
body: <p>Suppose I have the following <code>DataArray</code></p>
  <pre><code>arr = xarray.DataArray(np.arange(6).reshape(2,3),
                          dims=['A', 'B'],
                          coords=dict(A=['a0', 'a1'], 
                                      B=['b0', 'b1', 'b2']))
  </code></pre>
  <p>I want to iterate over the first dimension and do the following (of course I want to do something more complex than printing)</p>
  <pre><code>for coor in arr.A.values:
      print(coor, arr.sel(A=coor).values)
  </code></pre>
  <p>and get</p>
  <pre><code>a0 [0 1 2]
  a1 [3 4 5]
  </code></pre>
  <p>I am new to <code>xarray</code>, so I was wondering whether there was some more natural way to achieve this, something like</p>
  <pre><code>for coor, sub_arr in arr.some_method():
      print(coor, sub_arr)
  </code></pre>