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.

runner_dopair_grav() should abort if the distance between the cells is larger than the mesh smoothing scale.

runner_dopair_grav_pp() can now use the periodic boundary conditions. The L2L kernel is now always non-periodic as the shift should never cross a domain boundary.

Create the gravity pair tasks over the periodic boundary conditions. Make the long-range gravity task loop over the cells even over the periodic boundaries.

runner_dopair_grav_pp() can now use the periodic boundary conditions. The L2L kernel is now always non-periodic as the shift should never cross a domain boundary.

Create the gravity pair tasks over the periodic boundary conditions. Make the long-range gravity task loop over the cells even over the periodic boundaries.

runner_dopair_grav_pp() can now use the periodic boundary conditions. The L2L kernel is now always non-periodic as the shift should never cross a domain boundary.

Create the gravity pair tasks over the periodic boundary conditions. Make the long-range gravity task loop over the cells even over the periodic boundaries.

runner_dopair_grav_pp() can now use the periodic boundary conditions. The L2L kernel is now always non-periodic as the shift should never cross a domain boundary.

Create the gravity pair tasks over the periodic boundary conditions. Make the long-range gravity task loop over the cells even over the periodic boundaries.

Corrected the softening kernel. Modified the Plummer-equivalent constant. Created place-holder for order 3 softened M2L kernels.

Branch to a different, more accurate, calculation in the long-range that don't match the MAC any more.