title: Modify Eigen matrix diagonal
body: <p>I have an <code>Eigen::MatrixXd</code> and I need to modify the value of the elements in its diagonal. In particular I have another <code>Eigen::MatrixXd</code> with one single column and the same number of rows of the first matrix.</p>
  
  <p>I need to subtract to the diagonal of the first matrix the value of the elements of the second matrix.</p>
  
  <p>Example:</p>
  
  <pre><code>A
   1 2 3
   4 5 6
   7 8 9
  
  B
   1
   1
   1
  
  
  A'
   0 2 3
   4 4 6
   7 8 8
  </code></pre>
  
  <p>How can I do?</p>