title: numpy: is it possible to preserve the dtype of columns when using column_stack
body: <p>When I use <code>column_stack</code> to concatenate <code>NumPy</code> arrays, the <code>dtype</code> gets converted:</p>
  
  <pre><code>a = numpy.array([1., 2., 3.], dtype=numpy.float64)
  b = numpy.array([1, 2, 3], dtype=numpy.int64)
  print numpy.column_stack((a, b)).dtype
  &gt;&gt;&gt; float64
  </code></pre>
  
  <p>Is there a way to preserve the <code>dtype</code> of the individual columns?</p>