Merge branch 'gravity_speedup' into 'master'

configure.ac

examples/EAGLE_6/eagle_6.yml

src/cell.c

src/cell.h

src/engine.c

src/gravity.c

src/gravity/Default/gravity_iact.h

src/gravity_cache.h

src/gravity_derivatives.h

src/gravity_properties.c

src/gravity_properties.h

src/gravity_softened_derivatives.h

src/kernel_gravity.h

src/multipole.h

src/runner.c

src/runner_doiact_grav.h

src/scheduler.c

src/timers.c

src/timers.h

src/tools.c