Starts the animation timer. After calling this method the Animate method of all the entities is called periodically to allow the computation of the new entity position.

To animate group of entities the recommended approach is the following:

1. Create groups of moving objects (using Block class and adding entities to the Block.Entities collection)
2. Add blocks created at point 1) to the Blocks collection
3. Subclass the BlockReference class for each moving group of objects
4. Override the BlockReference.MoveTo() method of the class at point 3) and add the code to move the objects at that specific time frame
5. Override the BlockReference.IsInFrustum() method of the class at point 3) and call the base with the "center" transformed as the objects in point 4) to avoid undesired clipping
6. Add the block references created at point 4) to the Entities collection
7. Call StartAnimation() providing the time interval between each frame
8. Call StopAnimation() to stop the animation