title: use time without date as one dimension in xarray
body: <p>I have an xarray.DataArray that I successfully set one dim as time of one day.</p>
  <pre class="lang-py prettyprint-override"><code>tmp.dims
  ('ITEM', 'DATE', 'TIME', 'CODE')
  tmp.TIME
  &lt;xarray.DataArray 'TIME' (TIME: 15)&gt;
  array([datetime.time(14, 15), datetime.time(14, 16), datetime.time(14, 17),
         datetime.time(14, 18), datetime.time(14, 19), datetime.time(14, 20),
         datetime.time(14, 21), datetime.time(14, 22), datetime.time(14, 23),
         datetime.time(14, 24), datetime.time(14, 25), datetime.time(14, 26),
         datetime.time(14, 27), datetime.time(14, 28), datetime.time(14, 29)],
        dtype=object)
  tmp.TIME.values[0]
  datetime.time(14, 15)
  </code></pre>
  <p>but I cannot save this xarray do to the this error:</p>
  <pre><code>tmp.to_netcdf('/sdata/user/tsu/tmp/srpd.nc')
          *** ValueError: unable to infer dtype on variable 'TIME'; xarray cannot serialize arbitrary Python objects
  </code></pre>
  <p>Is it wrong to use datetime.time in this way?</p>