## Correct integration in time of the mesh forces

Complete rewrite of the time integration of the gravity mesh forces. This fixes the accuracy issues seen in the large-scale power-spectrum in cosmological runs.

This includes:

- Removing the mesh_force task.
- Introduce a global time-step for the mesh calculation.
- Compute the mesh forces when that time-step happens and not when we rebuild.
- Rewrite the operations in the kick tasks to simplify the with/without cosmology logic.
- Add the mesh accelerations to the gpart, remove the gravity acceleration from the xpart.
- Remove the alignment constraint on the gparts to reduce the memory footprint.
- Add an option to not compute the potentials.
- Remove the option to dither the particles at tree rebuilds. This was not the solution to the problem and was not implemented in an efficient way.

Todo:

- Clean-up the mesh time-step calculation.
- Deal with exact gravity force checks.
- Interpolation of the hydro velocities in snapshot.
- Deal with the time-step limiter.
- Deal with the time-step synchronization.
- Deal with the potential.
- Deal with star formation.
- Fix statistics.
- Documentation of the whole gravity scheme parameters.