Restore the early exit for cells that are far-away in the long-range force task. Restore the check that all particles interacted with everything in runner_end_force.

All-reduce all the top-level multipoles after a rebuild() to make sure we have all information in hands to construct the tasks.

Fixed the gravity bugs reported in #363. Need to find a good rebuild criterion for gravity-only runs.

Fix to #363. Drifting decision needs to use the same multipole information as the actual tree walk. Also fix some gravity_cache padding values.

Force compiler to report shadowed variables. Removed all shadowed variables. Added the function scheduler_activate_send() to simplify the unskipping of MPI sends.

Split the P2P and M2P into two separate (vectorizable) loops in the leaf-leaf gravity P-P interactions.