replace 'wait' with 'implicit' in scheduler_addtask, as we weren't using the former and could use the latter.

sort and drift tasks unlock all sorts and drifts above them, respectively, and activate tasks as is done in cell_unskip_tasks.

activate sorts and drifts in sub-cell tasks explicitly, store current values for h_max, dx_max, and dx_max_sort for consistency.

Disable particle cache when vectorisation is disabled.



See merge request !337

Dopair1 vectorisation merge

Summary
-------
- Contains a vectorised version of `runner_dopair1_density`
- `runners` now have two particle caches to hold cell ci and cell cj
- The `cache` has been updated to include `max_d` which is the maximum distance into the neighbouring cell
- `cache.h` contains more functions to read two cells into cache and to read a subset of particles from each cell into the cache


See merge request !320

Added softened gravity to the M2L kernels



See merge request !336

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.