Crucial +1 for the edge case where all particles in cell ci interact with at least one particle in cell cj.

Updated runner_doself1_density_vec and runner_dopair1_density_vec to align with Pedro's recent changes in DOPAIR1 and DOSELF1.

Merge branch 'dopair-vectorisation-merge' of gitlab.cosma.dur.ac.uk:swift/swiftsim into dopair-vectorisation-merge

Conflicts:
	src/runner_doiact.h

Rebuild criteria

Instead of rebuilding whenever the cell constraints or sort indices are violated, just re-sort in case of the latter.

This lead to some other changes:

 * Init tasks are gone, particle initialization is now handled in the drift tasks,
 * Drift tasks no longer act on the super-cell, but exist at every level similar to the sort tasks,
 * Fixed several dependency issues with sort and drift tasks,
 * Only drift/sort on-demand in the sub-cell tasks.

Marked as [WIP] as this has not yet been tested for MPI runs, or with gravity.

See merge request !327

Gravity multi dt



See merge request !333

Branch to a different, more accurate, calculation in the long-range that don't match the MAC any more.

Created functions to evaluate only W and dWdx separately along with vectorised versions. Also removed unneeded FMAs when kernel constants are zero.