Use intrinsics for vector operations as arithmetic overloading is not supported by the Intel compiler yet for AVX-512 vectors.

Conflicts:
	src/hydro/Gadget2/hydro_iact.h

Updated the way we converge towards the smoothing length. We now use a Newton-Raphson scheme. This applies to the Gadget-2 scheme.

Disabled GIZMO particle steering for vacuum particles. Disabled warning messages for unphysical values and pathological configurations.

Correctly initialise the signal velocity of the SPH particles in the Minimal and Pressure-Entropy cases.

Implemented the changes in ls that prevent gizmo crashes in cases where the mesh is seriously distorted

Initialize the signal velocity to be the particle's sound-speed. Prevents divisions by zero in cases where a particle has 0 neighbours.

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.

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

Replaced global variables by a new hydro_space struct that is part of space. This struct is then passed on to hydro_init_part, and used to initialize the Voronoi cells. For non SHADOWFAX_SPH schemes, this struct is empty. Replaced random_uniform declarations in some unit tests with the tools.h version. Added some more comments to 3D Voronoi code; it is now almost completely documented (meaning I almost completely understand it now).

Added a lot of comments to 3D Voronoi algorithm, fixed bug in testVoronoi3D. Disabled expensive checks. Some basic 3D moving mesh tests now work.

Added actual grid test to testVoronoi3D; degenerate case crashes. Started documenting 3D Voronoi algorithm to refresh my memory about how it works.

Fixed wrong box size in 2D Sod shock that caused SHADOWFAX_SPH to crash. Added flag in const.h to enable or disable cell motion steering. Made sure cell centroids have all their coordinates initialized.

Fixed bug in 2D Voronoi algorithm. Sod shock now works (with fixed cells). Moved gradient and slope limiter defines for SHADOWFAX_SPH to const.h.

Fixed small mistake in 2D Voronoi neighbour calculation. Added check for neighbour relations to unit test.

Realized I forgot to implement neighbour relations in 2D Voronoi algorithm. Implemented them. 2D Sod shock still doesn't work.

Fixed a bug in 2D Voronoi algorithm, added a lot of debugging output possibilities. 2D algorithm now works for 100 uniform random generators in a periodic box.

Finished first part of 2D Voronoi intersection algorithm. Now *only* need to write the second part. And maybe deal with degeneracies...

Cleaned up 1D Voronoi algorithm some more. Added SHADOWFAX_TOTAL_ENERGY flag and support for an isothermal EoS.