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.

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.