General speed-up improvements to the GIZMO interaction functions.

src/const.h

src/hydro/Gizmo/hydro_gradients.h

src/hydro/Gizmo/hydro_iact.h

src/hydro/Gizmo/hydro_slope_limiters_face.h

src/riemann/riemann_exact.h

src/riemann/riemann_hllc.h

src/riemann/riemann_trrs.h

src/riemann/riemann_vacuum.h