Wrong number of neighbours
Currently, we are assuming that
c->h_max is always the largest
h but this is not the case. Therefore the optimized version of the
iact is missing a few particles (less than one particle over 1000).
We can solve it with two different ways either we ensure to have a meaningful
c->h_max or we do not break the for loop when reaching
c->h_max (https://gitlab.cosma.dur.ac.uk/swift/swiftsim/blob/master/src/runner_doiact.h#L1004 and the one below)