title: Eigen extracting submatrix from vector of indices
body: <p>I have been googling for a while now, but cant find the answer to this simple question.</p>
  
  <p>In matlab i can do this:</p>
  
  <pre><code>rows = [1 3 5 9];
  A = rand(10);
  B = A(rows, : );
  </code></pre>
  
  <p>How do i do this in eigen? It does not seem like it is possible. The closest thing i have found is </p>
  
  <pre><code>MatrixXd a(10,10);
  a.row(1); 
  </code></pre>
  
  <p>,but I want to get multiple rows/cols. Another user has also asked the question here: <a href="https://stackoverflow.com/questions/26267194/how-to-extract-a-subvector-of-a-eigenvector-from-a-vector-of-indices-in-eige">How to extract a subvector (of a Eigen::Vector) from a vector of indices in Eigen?</a> , but I think there must some built in way of doing this because it is a really common operation I think. </p>
  
  <p>Thanks.</p>