title: How to refresh HelixToolkit ModelVisual3D content on pressed button?
body: <p>starting from a basic HelixToolkit example, I am able to render a mesh. In my .xaml file:</p>
  
  <pre><code> &lt;HelixToolkit:HelixViewport3D Name ="viewPort" ZoomExtentsWhenLoaded="True"&gt;        
   &lt;HelixToolkit:SunLight/&gt;
   &lt;!--The content of this visual is defined in MainViewModel.cs--&gt; 
   &lt;ModelVisual3D Content="{Binding Model}"/&gt; 
  </code></pre>
  
  <p>And in my .cs file:</p>
  
  <pre><code>Model3DGroup modelGroup = new Model3DGroup();            
  // [... add stuff to modelGroup  as children ]
  this.Model = modelGroup;
  </code></pre>
  
  <p>Now, I try to modify my rendered objects when a button is clicked. I tried to implement it through a callback function by updating the model. </p>
  
  <pre><code>public void testUpdate(){
         Model3DGroup newModelGroup = new Model3DGroup();            
         // [... add stuff to newModelGroup  as children ]
         this.Model = newModelGroup;
      }
  </code></pre>
  
  <p>This doesn't work: the shown objects are not those added to newModelGroup. How can I do that?</p>