Merge branch 'gizmo_speedup' into 'master'

src/Makefile.am

src/adiabatic_index.h

src/cell.c

src/const.h

src/hydro/Gizmo/hydro.h

src/hydro/Gizmo/hydro_gradients.h

src/hydro/Gizmo/hydro_gradients_gizmo.h

src/hydro/Gizmo/hydro_gradients_sph.h

src/hydro/Gizmo/hydro_iact.h

src/hydro/Gizmo/hydro_slope_limiters_cell.h

src/hydro/Gizmo/hydro_slope_limiters_face.h

src/hydro/Gizmo/hydro_unphysical.h

src/hydro/Gizmo/hydro_velocities.h

src/riemann/riemann_checks.h

src/riemann/riemann_exact.h

src/riemann/riemann_exact_isothermal.h

src/riemann/riemann_hllc.h

src/riemann/riemann_trrs.h

src/riemann/riemann_vacuum.h

src/sign.h