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.

1D and 2D moving mesh algorithm

I have added a new hydro scheme, `SHADOWFAX_SPH`, that uses a finite volume method on a moving mesh. The scheme is very similar to `GIZMO_SPH`, except that during the density loop an actual Voronoi cell is constructed for every particle, which is then used to get a volume to update the primitive quantities, and faces through which fluxes are exchanged. There are also some small changes in the way gradients are computed, and in the way the velocity of the particles is set.

The Voronoi cell construction is completely different for 1D, 2D, and 3D, and currently only 1D and 2D have a stable implementation. 3D should compile, but does not produce reliable results (yet). However, I think it would be good to merge this in already, to prevent it from drifting too far apart from `master`.

The current implementation is hydro only, but it should be very straightforward to add gravity, similarly to the way it is done in `GIZMO_SPH`.

Below the results for the 2D Sod shock and Sedov blast test (the noise on the density is caused by the fact that the initial conditions contain mass and not density). The 2D Noh test crashes because of time stepping issues; I think we really need a time step limiter to make it work properly.

![SodShock](/uploads/641796ae26cf045be5ce824bb698b77a/SodShock.png)

![Sedov](/uploads/96be4bbeb7635c329b78f89e0e79239c/Sedov.png)

See merge request !321

Added some more comments to 3D algorithm, and fixed a bug that affected some of the degenerate cases.