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.

Fixed test125cells when using the Wendland kernels. Neighbour number tolerance was not large enough.

Added GIZMO_SPH option in const.h. Made sure the resulting code compiles. It probably does not work correctly though.

Updated 'make check' scripts with newer tolerances for the Intel compiler and made the scripts choke if files are missing