system potential computation
@elena: In the main loop, the system potential is computed at the begining of the loop. But it is only used a few lines later in the velocityStep
(ex subStep1). In the mean time, the planets are advanced.
If we compute it where it's used (after the planet are advanced) the results changes a little bit.
I can understand why they change (planets are not exactly at the same places) but is the result "worse" if we call it only where needed ? Memory usage would be better that way.