TYPE: POP
TITLE: RBDUnify POP

The RBDUnify POP is used to unite multiple RBD particles into one
unified particle. RBD particles that are unified in this way are no
longer treated independently by the RBD solver. Instead, information
about the sub-particles is controlled by the unified particle.

Parameters:
Birth
    Activation          - Turns the POP on and off		/activate
    Source Group        - Subset of points to unify
    Birth Group         - Group to contain birthed points
    Preserve Birth Group - Add particles to group instead of clearing group first
    Collision Event     - Event triggered by rigid body collisions
    Collision Group     - Group of colliding rigid body particles
    Preserve Collision Group - Do not remove entries from collision group
                               after each frame
    Merge Input Particle Groups - Make unified particle a member of any group
                                  that any sub-particle belongs to
Add Attributes
    Add ID Attributes          - Add ID and parent attributes
    Add Speed Attribute        - Add speed attribute (length of velocity vector)
    Add Num Hit Attribute      - Number of times collided
    Add Hit Time Attribute     - Time of collision
    Add Hit Position Attribute - Where collision occurred
    Add Hit Normal Attribute   - Normal where collision occurred
    Add Hit Force Attribute    - Force of collision 
