Remove unnecessary dependency between sort and self/density tasks.

See merge request !461

Improvements to i/o and parallel-i/o

See merge request !460

Corrected typo in Minimal-SPH implementation that prevents neighbour number convergence in some cases.

Make sure the code compiles with vectorization switched on and using a different hydro scheme from Gadget-2.

Use a different criterion for gravity or hydro to construct the proxies corresponding to a given cell.

Fixed a moronic bug. Unskipping MPI sends and recvs for gravity should not be done *only* for local pairs. Obviously....

Real problem was a 28 * k, instead of k < 26 *, but clear up the logic and fix some edge cases where the range could over/underflow a bit, also add a debugging check to spot this issue next time

Removed unnecessary dependency between the out-going gravity ghost and the self-grav and pair-grav tasks.