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.

Repaired GIZMO_SPH after the time integration changes that had been made.

The most important change is the addition of a new method, `hydro_timestep_extra`, that is called after the new time step for a particle has been computed, and that passes the physical time step for the next step to the particle.
In `GIZMO_SPH`, this method is used to
 - store the physical time step in `p->force.dt`. This value is used during the force loop to exchange fluxes between particles
 - reset a new `p->force.active` flag to an inactive value (`0`). This flag is set to an active value (`1`) in `hydro_init_part`, and is used during the force loop to decide whether or not to exchange flux with particle `j` in the asymmetric version of the interaction method.

For all other hydro schemes, this method is empty.

Other changes:
 - made sure `benchmarkInteractions` compiles with `GIZMO_SPH`
 - added a compilation flag, `GIZMO_FIX_PARTICLES`, that disables particle movement. If possible, this should be replaced by a corresponding parameter
 - added some sanity checks on density and pressure values, and a proper treatment of vacuum in the gradient prediction step

This version successfully runs most of the hydro examples (including the Sod tests, Sedov blasts, Kelvin-Helmholtz test, square tests, Gresho vortex), but fails to run the 2D and 3D Noh problem (1D works).

See merge request !315

Streamlined GIZMO_FIX_PARTICLES usage (tried to add parameter, but this does not work, as space_init has no access to the hydro_properties). Moved conserved variable initialization to hydro_first_init_part, and removed it from hydro_convert_quantities. Added some sanity checks to maintain positive density and pressure.

Cleaned up Gizmo fixes. Added a hydro_timestep_extra method and added an empty version for all hydro schemes.

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.