title: Eigen3 Flatten Matrix as Vector
body: <p>In MATLAB, I can do the following</p>
  
  <pre><code>A = [1 2 3; 4 5 6];
  A(:)
  </code></pre>
  
  <p>to get:</p>
  
  <pre><code>ans =
  1
  4
  2
  5
  3
  6
  </code></pre>
  
  <p>How would I do this with an Eigen3 Matrix?</p>