title: write matrix to file in eigen?
body: <p>I am trying to learn C++ with the Eigen library.</p>
  
  <pre><code>int main(){
      MatrixXf m = MatrixXf::Random(30,3);
      cout &lt;&lt; "Here is the matrix m:\n" &lt;&lt; m &lt;&lt; endl;
      cout &lt;&lt; "m" &lt;&lt; endl &lt;&lt;  colm(m) &lt;&lt; endl;
      return 0;
  }
  </code></pre>
  
  <p>How can I export <code>m</code> to a text file (I have searched the documentations 
  and have not found mention of an writing function)?</p>