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.

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

Use the multipole acceptance criterion to build the long-range interactions and not just the geometry of the mesh.

Simplify notation by passing only the field tensor rather than the whole multipole structure to the gravity functions.

Check whether particles or cells are active via an inlined function rather than via a direct read of engine->ti_current