Scheduler doforcesplit off

The scheduler_doforcesplit flag will make the scheduler split tasks that have too many particles even if the smoothing lengths inside the cells are too large. This means that interactions will be generated with cells outside of the 27 direct neighbours.

Switching this off for now as it causes massive problems with engine_addlinks, which assumes a maximum number of density/force tasks per cell. I have added a comment warning users to this effect, should re-design engine_addlinks at some point to avoid this type of problem.

Note that I don't want to remove this option permanently as it could be useful in cases with density gradients >2.