- Particle-particle effectors we're quite unstable and not accurate at all. Now this...

- Particle-particle effectors we're quite unstable and not accurate at all. Now this should be fixed (especially with other integrators than euler) as the needed inter-frame effector particle positions are interpolated properly from the current and previous positions (previously only the most recent position was used).
- In practice this removes the dependency of particle simulations from the update order of objects and different particle systems inside objects.
- As a nice side effect out of this we also get fully correct birth positions for "near reactor particles" (previously for example smoke trail reactor particles were not born smoothly along the target particles path).