title: add new coordinates to an xarray.DataArray
body: <p>Assume the following code, which has an <code>xarray.DataArray</code> with two dimensions and a coordinate:</p>
  
  <pre class="lang-py prettyprint-override"><code>import numpy as np
  from xarray import DataArray
  
  data = np.random.rand(10, 4)
  f_names = ['a', 'b', 'c', 'd']
  sample_weights = np.random.rand(10)
  rows = list(range(len(data)))
  coords={'samples': rows,
          'features': f_names,
          'sample_weights': ('samples', sample_weights)}
  xdata = DataArray(data, coords=coords,
                    dims=['samples', 'features'])
  
  subset = xdata[::2]
  </code></pre>
  
  <p>Now I want to add another coordinates, like <code>alternate_sample_weights</code> to <code>subset</code>. I try:</p>
  
  <pre class="lang-py prettyprint-override"><code>subset.assign_coords(alternate_sample_weights=np.zeros(5)
  </code></pre>
  
  <p>which results in the following error:</p>
  
  <pre class="lang-py prettyprint-override"><code>ValueError: cannot add coordinates with new dimensions to a DataArray
  </code></pre>
  
  <p>The <a href="http://xarray.pydata.org/en/stable/generated/xarray.DataArray.assign_coords.html#xarray.DataArray.assign_coords" rel="noreferrer">API documentation</a> is pretty sparse, not sure what I'm doing wrong.</p>