Atomic gravity and time-step limiter

Changes:

• Use atomic operations for the gravity tasks that update gpart,
• 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).