title: Error mixing types with Eigen matrices
body: <p>There was no quick find answer that I could see on stack for this problem so I thought I would add one. </p>
  
  <p>Say I have the following example code from the c++ Eigen Library:</p>
  
  <pre><code>Eigen::Matrix4d m1;
  Eigen::Matrix4f m2;
  m1 &lt;&lt; 1, 2, 3, 4 ... 16
  m2 = m1; //Compile error here.
  </code></pre>
  
  <p>I get a compile error on the final line that boils down to this:</p>
  
  <pre><code>YOU_MIXED_DIFFERENT_NUMERIC_TYPES__YOU_NEED_TO_USE_THE_CAST_METHOD_OF_MATRIXBASE_TO_CAST_NUMERIC_TYPES_EXPLICITLY
  </code></pre>
  
  <p>What is an easy way to fix this?</p>