Merge branch 'master' into Self_gravity_BH

src/cell.h

src/engine.c

src/runner.c

src/runner.h

src/runner_doiact.h

src/runner_doiact_grav.h

src/timers.h

tests/test27cells.c

tests/testPair.c

tests/testSPHStep.c

tests/testTimeIntegration.c