set angular velocity

[Mathieu Leclaire]

I cache an ICE tree with particles created in the modeling stack, so I 
have no linear and angular velocity. I need these values for proper 
motion blur.

Linear velocity is easy to calculate: subtract previous frame position 
by current position and multiply the resulting vector by the inverse of 
the Frame Step. That gives me proper linear motion blur.

Now I need to do the same thing for AngularVelocity, but I can't seem to 
find the right way to calculate it. The motion blur on the particles are 
always too weak compared to the same animated objects. How can I 
calculate the appropriate AngularVelocity value?

-Mathieu