Stars smoothing length over MPI

The stars over MPI are implemented for the density and the feedback. This implementation is not able to deal with new stars in cells without previous stars. I can do it when I will come back from holidays if this merge request is still open.

Here is the task graph for the DwarfGalaxy example:

For the communications, I am sending only the part before the density and only the spart between the density and the feedback.

Here is the number of neighbors found in the feedback task for the DwarfGalaxy:

In orange, you have the number of neighbors found without MPI and in blue with MPI.

I was not able to test it with EAGLE_6. I can do it after the holidays.