title: How to get an Eigen vector from an Eigen matrix column/row?
body: <p>The thing I want to do is this:</p>
  <pre><code>MatrixXd M;
  // Whatever assigned to `X`
  VectorXd V = M(1);
  </code></pre>
  <p>Eigen does not support this. Is there a way to retrieve an Eigen vector from an Eigen matrix?</p>