title: Mapping row-major array to column-majored Eigen Matrix
body: <p>I want to map from a C-type array to a Column majored Eigen matrix.</p>
  
  <p>The mapping itself is using the RowMajor type,</p>
  
  <p>so I tried</p>
  
  <pre><code>std::vector&lt;double&gt; a(9);
  double *p= a.data();
  Eigen::MatrixXd M=Eigen::Map&lt;Eigen::Matrix&lt;double, 3, 3, Eigen::RowMajor&gt;&gt;(p)
  </code></pre>
  
  <p>I got what I expected(the order of M.data()), however, if the dimension(3) in the template is not known at compile time, this method doesn't work... any solution? </p>