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.

doc/RTD/source/SubgridModels/EAGLE/index.rst

@@ -62,12 +62,21 @@ EAGLE run, that section of the parameter file reads:
      Jeans_density_threshold_H_p_cm3: 0.1       # Physical density above which the EAGLE Jeans limiter entropy floor kicks in, expressed in Hydrogen atoms per cm^3.
      Jeans_over_density_threshold:    10.       # Overdensity above which the EAGLE Jeans limiter entropy floor can kick in.
      Jeans_temperature_norm_K:        8000      # Temperature of the EAGLE Jeans limiter entropy floor at the density threshold, expressed in Kelvin.
-     Jeans_gamma_effective:           1.3333333 # Slope the of the EAGLE Jeans limiter entropy floor
+     Jeans_gamma_effective:           1.3333333 # Slope of the EAGLE Jeans limiter entropy floor
      Cool_density_threshold_H_p_cm3:  1e-5      # Physical density above which the EAGLE Cool limiter entropy floor kicks in, expressed in Hydrogen atoms per cm^3.
      Cool_over_density_threshold:     10.       # Overdensity above which the EAGLE Cool limiter entropy floor can kick in.
      Cool_temperature_norm_K:         8000      # Temperature of the EAGLE Cool limiter entropy floor at the density threshold, expressed in Kelvin.
-     Cool_gamma_effective:            1.        # Slope the of the EAGLE Cool limiter entropy floor
-
+     Cool_gamma_effective:            1.        # Slope of the EAGLE Cool limiter entropy floor
+
+SWIFT will convert the temperature normalisations and Hydrogen number
+density thresholds into internal energies and densities respectively
+assuming a neutral gas with primoridal abundance pattern. This implies
+that the floor may not be exactly at the position given in the YAML
+file if the gas has different properties. This is especially the case
+for the temperature limit which will often be lower than the imposed
+floor by a factor :math:`\frac{\mu_{\rm neutral}}{\mu_{ionised}}
+\approx \frac{1.22}{0.59} \approx 2` due to the different ionisation
+states of the gas.
 
 Note that the model only makes sense if the ``Cool`` threshold is at a lower
 density than the ``Jeans`` threshold.