Merge branch 'master' into doself2-vectorisation

examples/MultiTypes/multiTypes.yml

examples/parameter_example.yml

m4/ax_gcc_archflag.m4

src/debug.c

src/debug.h

src/engine.c

src/engine.h

src/gravity_cache.h

src/runner.c

src/runner_doiact.h

src/runner_doiact_grav.h

src/scheduler.c

src/scheduler.h