title: Add row and column at zero position in matrix Eigen
body: <p>I've got the matrix for example : C(400,400)</p>
  
  <p>and I would like to grow this matrix with one vector: row and column of this matrix at the begin-index 0 of the matrix for example :</p>
  
  <p>Matrix C:</p>
  
  <pre><code> 3 2 5 
   4 5 6
   7 8 20
  </code></pre>
  
  <p>my new vector :  25 5 6 8</p>
  
  <p>Result :</p>
  
  <pre><code>  25 5 6 8
    5  3 2 5 
    6  4 5 6
    8  7 8 20
  </code></pre>
  
  <p>Whats the best way in Eigen to do this? Use <code>.resize()</code>, and <code>.set</code>? Thanks for help </p>