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