title: XGBoost: Convert dmatrix into a numpy.array
body: <p>I'd like to inspect the <code>DMatrix</code> object. The <a href="http://xgboost.readthedocs.io/en/latest/python/python_api.html#xgboost.DMatrix" rel="noreferrer">documentation mentions</a> the methods to get the number of rows and columns and also the slice at each row:</p>
  
  <pre><code>dmatrix.slice(index)
  </code></pre>
  
  <p>But I'd like to do more complex transformations using numpy. Is there a simple way to convert the <code>DMatrix</code> to a numpy array?</p>