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

Broke one of my own rules by doing too much in a single commit: implemented runner_extra_ghost, moved gradient calculations for both gradient types to new gradient loop, fixed bug in engine.c involving the number of links (which is larger with an extra loop), streamlined gradient methods, documented most GIZMO specific files. Second loop works.

Added volume to Gizmo/hydro_io. Added volume unit conversion. Isolated gradients calculations in separate files.

Removed rho_dh from Gizmo/hydro_part. Commented out USE_GRADIENTS and PER_FACE_LIMITER in Gizmo/hydro_iact.

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