Fix a faulty debugging check in the new gravity code that triggered when using an empty external potential

Hide the atomics into a layer of functions that can be used to update the particles atomically or not in a transparent way

accumulated from P2P, M2P, M2L and PM for each particle individually (not just
the combination of the four).

It also counts the interactions from each force contributor individually.

Added a new particle type for DM boundary particle in zooms. Created a new flavour of gravity: multiple-sotening.

Added direct calculation of the mesh forces when checking accuracy. Use higher-order CIC interpolation when translating the mesh back to the field tensors.

Pass an extra argument to the function returning the particle's softening. Removed the field from the gpart themselves.