Compute the total mass in the system and add a variable to the gpart to record how much mass they interacted with via self-gravity.

Added function to drift the multipole of a given cell without updating its progenies.

Upgraded version number to reflect correct treatment of hydro in multi-dt and MPI as well as the inclusion of star particles.

Timestep order

This is my last batch of changes regarding time integration.
 
 - Additional checks here and there that the particles are synchronized. Especially gpart.
 - Re-arranged the time-step sequence and simplified the logic in engine_step().
 - Moved the decision of repartitioning from the main() to engine_step().

The re-arrangement of the time-step sequence allows for perfect energy conservation even when we rebuild, repartition or dump snapshots. Previously that was not the case as a spurious drift had to be introduced. 

See merge request !314

Properties of the self-gravity scheme are now read from the YAML file. They are printed to stdout and to snapshots.

Properties of the self-gravity scheme are now read from the YAML file. They are printed to stdout and to snapshots.