Merge branch 'master' into space_rebuild

examples/main.c

m4/ax_cc_maxopt.m4

src/Makefile.am

src/active.h

src/cell.c

src/cell.h

src/debug.c

src/debug.h

src/engine.c

src/hydro/Default/hydro.h

src/hydro/Gadget2/hydro.h

src/hydro/Gizmo/hydro.h

src/hydro/Minimal/hydro.h

src/hydro/PressureEntropy/hydro.h

src/runner.c

src/runner.h

src/runner_doiact.h

src/runner_doiact_grav.h

src/scheduler.c

src/scheduler.h