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

Prevent users from using an FFT mesh with more than 2^31 cells, which would cause an integer over-flow in the loops.

Use the threadpool and atomic operations to parallelize the assignment of gparts to the density mesh.

Do not access the space any more in the gravity calculation functions. All relevant information are stored in the pm_mesh structure.