Use the primoridal Helium abundance to compute the Hydrogen mass fraction. Optionally read the temperature transition from the YAML parameter file.

Make reports a little less specific

Add a new parameter to the gravity calculation to control the frequency of the tree rebuild using a fraction of the number of gpart updates.

Improve the way we define the time of the first snapshot and statistics in cosmology and non-cosmology modes.

Fix a problem distributing the final partition, that was broken in the previous commit

Read the minimally allowed temperature from the parameter file and convert it to a minimal internal energy per unit mass.