change space_getcell to space_getcells, can pick up more than one cell at a time.

As discussed in #255. Have only checked that it works, haven't done any benchmarks, but this can only make things faster 😉 

See merge request !300

Call vector

Call vectorised version of `runner_doself1_density` when vectorisation is turned on.

See merge request !302

Particle caching

Summary
-------
- Implements a vectorised version of `runner_doself1_density()`. 
- Implements a particle cache on each thread and allocates the memory `in engine_init()`
- Modified `test27cells.c` so that particles that have been interacted with vectors are checked manually against a serial interaction
- Implemented another vectorised version of `kernel_deval()` which calculates the kernel for two particles.
- Fixes a bug in `runner_iact_vec_force` and `runner_iact_nonsym_vec_force` when calculating the SPH term

`test27cells` will fail because of the thresholds set for the diff, I have attached the differences in: [brute_force_h_1.2348.dat](/uploads/8e32fd88cee93bd429bc57c64ab16b4d/brute_force_h_1.2348.dat)
[swift_dopair_h_1.2348_avx_doself1_loop.dat](/uploads/4c61c74b5d4a7aef8a2c0d01cdc17e9e/swift_dopair_h_1.2348_avx_doself1_loop.dat)

@nnrw56 @pdraper



See merge request !287

Check that the particles interacting in the SELF or PAIR tasks have been properly drifted to the current time