Only construct the send/recv tasks for a hydro/gravity when required by the type of cell in the proxy.

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....