Merged requested to merge atomic_gravity into master
- Use atomic operations for the gravity tasks that update
- Use locks in the gravity tasks that update multipoles,
- Use atomic operations for the limiter task,
- Do no lock the tree for the gravity and limiter tasks,
- Add an atomic max for chars,
- Minor changes to the parameter files,
- Add an option in the configuration script to revert to the old behaviour.
Implements #359 (closed).