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.

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