title: How to make sub groups in sonata admin form
body: <p>In Sonata's <code>AdminBundle\Mapper\BaseGroupedMapper.php</code>, I saw a code example:</p>
  
  <pre><code>    public function with($name, array $options = array())
      {
      /*
       * The current implementation should work with the following workflow:
       *
       *     $formMapper
       *        -&gt;with('group1')
       *            -&gt;add('username')
       *            -&gt;add('password')
       *        -&gt;end()
       *        -&gt;with('tab1', array('tab' =&gt; true))
       *            -&gt;with('group1')
       *                -&gt;add('username')
       *                -&gt;add('password')
       *            -&gt;end()
       *            -&gt;with('group2', array('collapsed' =&gt; true))
       *                -&gt;add('enabled')
       *                -&gt;add('createdAt')
       *            -&gt;end()
       *        -&gt;end();
       *
       */
  </code></pre>
  
  <p>Unfortunately, I get an error if I add a group first and THEN add tabs. I want my form to have a main simple form (firstname, etc...), and then tabs  under it to list the entity relation forms (onetomany...) tab by tab to keep it clean. Unfortunately, I get this error:</p>
  
  <pre><code>New tab was added automatically when you have added field or group. You should close current tab before adding new one OR add tabs before adding groups and fields.
  </code></pre>
  
  <p>Does anyone know how to make this work ? Or this was 2 separated examples? I would like if possible to avoid having pure tabs and therefore not being able to have a part of my form constantly visible.</p>