Merge branch 'atomic_gravity' into 'master'

.gitignore

configure.ac

examples/EAGLE_ICs/EAGLE_12/eagle_12.yml

examples/EAGLE_ICs/EAGLE_25/eagle_25.yml

examples/EAGLE_ICs/EAGLE_50/eagle_50.yml

src/Makefile.am

src/accumulate.h

src/atomic.h

src/cell.c

src/engine.c

src/gravity/MultiSoftening/gravity.h

src/gravity/Potential/gravity.h

src/gravity_cache.h

src/mesh_gravity.c

src/multipole.h

src/runner_doiact_grav.c

src/task.c

src/timestep_limiter_iact.h

tests/Makefile.am

tests/testAtomic.c