title: How to define Vector with template using Eigen library?
body: <p>I know Eigen use Matrix to represent vector, but for templatrized vector, what I know is to use something like</p>
  
  <pre><code>Eigen::Matrix&lt;T,3,1&gt; v;
  </code></pre>
  
  <p>Is there something like
      <code>Eigen::Vector&lt;T,3&gt;</code> for use?</p>