title: converting pandas dataframe to xarray dataset
body: <pre><code>   Unnamed: 0       index    datetime  ...   cVI     Reg       average_temp
  0           0  2000-01-01  2000-01-01  ...   NaN  Central           -5.883996
  1           1  2000-01-02  2000-01-02  ...   NaN  Central           -6.715087
  2           2  2000-01-03  2000-01-03  ...   NaN  Central           -6.074254
  3           3  2000-01-04  2000-01-04  ...   NaN  Central           -4.222387
  4           4  2000-01-05  2000-01-05  ...   NaN  Central           -0.994825
  </code></pre>
  <p>I want to convert the dataframe above to an xarray dataset, with <code>datetime</code> as the index. I do this:</p>
  <pre><code>ds = xr.Dataset.from_dataframe(df)
  </code></pre>
  <p>but I am not able to get the <code>datetime</code> column as index. How do I do that?</p>