Moved the sorting entry structure to a separate file following @jwillis' work in the vectorisation branch.

Voronoi tests work now.

I think I added all missing files used by `SHADOWFAX_SPH` and `GIZMO_SPH` to `Makefile.am`.

I have also introduced a tolerance for geometric tests in the 2D Voronoi algorithm, which fixes round off issues that caused `testVoronoi2D` to fail if compiled with optimizations enabled.

This fixes #282.

See merge request !325

Gravity multi dt

This fully implements self-gravity bar the periodic task over MPI. 

Could you please check that is does not break anything on the hydro side ? 

If you are interested in running with self-gravity I'd recommend using the updated EAGLE_12 case and run with `-D -G`. 

My next steps is to work on the formal requirement of using `-D` alongside the `-G`. I also need to finish working out the automated scripts to generate code for the higher-order multipoles but that is orthogonal to the integration of the code into the master.

See merge request !324

Added missing SHADOWFAX_SPH and GIZMO_SPH files to src/Makefile.am. Removed unused counter from riemann_solve_brent().

Introduced a tolerance for the 2D Voronoi algorithm to handle round off problems. The 2D Voronoi test now works for both the optimized and the non-optimized version of the code.