Added SWIFT code group

Adds ```Code/Code = "SWIFT"``` in the output files which can be used as a check to see if the output was generated by SWIFT.



See merge request !439

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.

Fixed bug where max_index_i could be count_j instead of count_j - 1. Also fixed bug where an exit iteration of 0 prevents the inner loop from executing. For dopair2.

Added a test which interacts a pair of cells with only one particle each. Disabled error in shuffle_particles.

Fixed bug where max_index_i could be count_j instead of count_j - 1. Also fixed bug where an exit iteration of 0 prevents the inner loop from executing.

Take the maximum between hi and hj_max when working out which particles to skip. This ensures all interactions are found. Sedov blast now runs correctly with runner_dopair2_force_vec.

Pad cache with better fake particles so that they won't be in range of real particles. Sedov blast now runs to completion but timesteps don't match serial output.

Also send dx_max_part and dx_max_gpart alongside ti_end_min at the end of a time-step when running with MPI

Implements the solution to #367. Instead of just sending the time-step we send the distances moved by any part and gpart in that cell. 

I have also done some cleaning up around the packing and unpacking and improved the documentation.

See merge request !434

Correct vec neighbour find

Fixes neighbour find bug in the vectorised version of the density interactions.

See merge request !435

Added an extra test case to testActivePair.sh to include the RNG seed that creates a vectorised neighbour find bug.

Also send dx_max_part and dx_max_gpart alongside ti_end_min at the end of a time-step when running with MPI.