title: get list of keys in xarray dataset
body: <p>I'm adding variables to an xarray Dataset, and when I'm done, I want to extract a list of the keys, but I couldn't find a way to do that.</p>
  
  <pre><code>d = xr.Dataset()
  d['key1'] = [1, 2]
  d['key2'] = [3, 4]
  </code></pre>
  
  <p>My desired result is</p>
  
  <pre><code>keylist = ['key1', 'key2']
  </code></pre>