Pre-declare the sump structure to avoid warnings when the whole dumping mechanism is not available. Remove the '-malign-double' flag from the clang  compilation line.

Also detect whether the current POSIX implementation offers the optional function posix_fallocate(). If not, switch off the particle streamed logging.

Added information about using SWIFT on Apple devices as well as wrapping dump.c in an ifdef __APPLE__

Implement simple thread barriers to be used on systems where the POSIX implementation does not provide them. We default to the POSIX ones when detected at configure time.

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.