title: Fast way to convert upper triangular matrix into symmetric matrix
body: <p>I have an upper-triangular matrix of <code>np.float64</code> values, like this:</p>
  
  <pre class="lang-py prettyprint-override"><code>array([[ 1.,  2.,  3.,  4.],
         [ 0.,  5.,  6.,  7.],
         [ 0.,  0.,  8.,  9.],
         [ 0.,  0.,  0., 10.]])
  </code></pre>
  
  <p>I would like to convert this into the corresponding symmetric matrix, like this:</p>
  
  <pre class="lang-py prettyprint-override"><code>array([[ 1.,  2.,  3.,  4.],
         [ 2.,  5.,  6.,  7.],
         [ 3.,  6.,  8.,  9.],
         [ 4.,  7.,  9., 10.]])
  </code></pre>
  
  <p>The conversion can be done in place, or as a new matrix. I would like it to be as fast as possible. How can I do this quickly?</p>