DamageImpact and Impulse energy fix

This moves to a more physically correct computation of damage and impulse created on collision of 2 objects, based on the formula of the partly elastic impulse.

Before this, bullets colliding with a heavy object got way too much damage, for example.
Well, yes/no. A collision frees energy (in Joule) which is applied to both collisionparties. The velocity-effect it has on them is determined by the mass, but this is not the job of the ImpactGenerator. 
I find this distribution correct and remember putting a bit of thought into it when I originally implemented it, that's why I guess it has some logic to it ;)