Added SWIFT code group to the output files to enable a quick check from other programs if the output was generated by SWIFT

Created a branching function for DOPAIR2 to only call the vectorised function when vectorisation is enabled and call the serial version when corner cells interact. Moved some debug statements inside the branching functions.

Pad particle caches with sensible values for the smoothing length to prevent floating point exceptions.

runner_dopair2_force_vec now runs correctly without looping over all particles. max_index have been populated using the max between hi and hj_max.

pairs_all_force now only updates active particles. Removed error message in shuffle_particles for arrays of length 1.