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

Updated the tests to use the new cell properties for sorting and not the old ones. All tests now run smoothly. Make sure the xparts array actually exist in a cell before reinitialising its particles.

Updated the scripts running the tests to actually crash if they fail at any iteration and not just on the last one. Code formatting. Do not compile the FFT functions if FFTW is not found.

No limit on h in test125cells. Add more digits to the output. Perturbed version of the test125cells now works.

Conflicts:
	tests/test125cells.c

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).

3D SHADOWFAX_SPH compiles again (but does not really work). Re-introduced global variables to store the box size.

Constructing a 3D voronoi grid now seems to work. Still need to find a way to correctly initialize the box.

Wrote 1D Voronoi cell generation algorithm and corresponding unit test. Algorithm is not coupled into swift yet.