title: Rotation matrix to vector with angle
body: <p>Using <code>HelixToolkit</code> in my code i have a rotation matrix (3x3) like : </p>
  
  <pre><code>UX  VX  WX
  
  UY  VY  WY
  
  UZ  VZ  WZ
  </code></pre>
  
  <p>And i want to rotate a <code>GeometryModel3D</code>. I found the <code>RotateTransform3D</code> and i need a Vector3D with an angle : </p>
  
  <pre><code>// Create and apply a transformation that rotates the object.
  RotateTransform3D myRotateTransform3D = new RotateTransform3D();
   AxisAngleRotation3D myAxisAngleRotation3d = new AxisAngleRotation3D();
  **myAxisAngleRotation3d.Axis = new Vector3D(0, 3, 0);
  myAxisAngleRotation3d.Angle = 40;**
  myRotateTransform3D.Rotation = myAxisAngleRotation3d;
  // Add the rotation transform to a Transform3DGroup
  Transform3DGroup myTransform3DGroup = new Transform3DGroup();
  myTransform3DGroup.Children.Add(myRotateTransform3D);
  //ajoute the transformation to the model
  model3D.Transform = myTransform3DGroup;
  </code></pre>
  
  <p>How can i calculate the vector and the angle from my rotation matrix ?</p>