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 swift_dopair_h_1.2348_avx_doself1_loop.dat

@nnrw56 @pdraper