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.

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).