Added a maximal smoothing length that can be used to prevent particles from becoming too big in vacuum cases near the box edges. Will break the accuracy of the hydrodynamic scheme but is necessary in some situations.

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.