Class: AnimationObjectGroup
AnimationObjectGroup()
A group of objects that receives a shared animation state.
Usage:
 - Add objects you would otherwise pass as 'root' to the
   constructor or the .clipAction method of AnimationMixer.
 - Instead pass this object as 'root'.
 - You can also add and remove objects later when the mixer
   is running.
Note:
   Objects of this class appear as one object to the mixer,
   so cache control of the individual objects must be done
   on the group.
Limitation:
 - The animated properties must be compatible among the
   all objects in the group.
 - A single property can either be controlled through a
   target group or directly, but not both.