Fixed bug where accelerations wouldn't update.

The accelerations stayed at zero.
The cause was that the GravitationSystem used planet A's acceleration
as B's acceleration. As these were the only two planets, their
accelerations were supposed to be the opposite of the other.
A received both accelerations: +1-1 =0.
Futher optimized GravitationSystem.

For each planet-pair:
trigonometry were calculated separatly, but used the same angle.
Minor.

Used edited a comment.