Stars use sort

In this branch, I am optimizing the dopair functions for the stars. They are about 100 times faster than the previous version on EAGLE_6. The global speedup is about 2x.

I discovered a bug in the optimized version of the hydro. Here https://gitlab.cosma.dur.ac.uk/swift/swiftsim/blob/master/src/runner_doiact.h#L1003 we cannot use this exit condition. hi_max is not always the max, therefore we can skip a few particles (quick estimation of about 1 missed neighbors every 10'000 particles).

I am not solving it yet (will be in a separate merge request), but the stars are not reproducing this error. Due to this error, the stars sort is not as useful as expected, therefore I am also removing the sorts.

@matthieu, I am also fixing a few checks from your branch.

Currently I am requesting to merge with mpi_stars2_matthieu, but we can wait a bit longer and do it directly to the master.