title: Return double from vector multiplication with Armadillo C++
body: <p>Do you know why I cannot store in a double the result from a vector multiplication ?</p>
  
  <pre><code>double A = rowvec({1,3,4})*vec({5,6,7});
  </code></pre>
  
  <p>It gives "no suitable conversion function from "const arma::Glue"... to "const double" exists.</p>
  
  <p>Still that matrix vector multiplication gives a double.
  How can I get around ?</p>
  
  <p>Thank you!</p>