Merge branch 'master' into chemistry

examples/EAGLE_100/eagle_100.yml

examples/check_ngbs.py

examples/parameter_example.yml

src/cache.h

src/cell.c

src/cooling/grackle/cooling.h

src/debug.c

src/engine.c

src/partition.c

src/periodic.h

src/runner.c

src/runner_doiact.h

src/runner_doiact_vec.c

src/runner_doiact_vec.h

src/scheduler.c

src/space.c

src/space.h

src/timeline.h

tests/test27cells.c