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

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.

Don't lock the tree for the grav_init task. Don't call the unsorted pairs at all. Skip the entire zeroing loop when no xparts in runner_do_sort()

Updated kernel_deval_1_vec and kernel_deval_2_vec to compute both branches of the Cubic Spline kernel and combine the result with masks.