Cleaned up GIZMO gravity a bit. Still doesn't work. Added a gradient interpolation limiter that should solve the Noh problem.

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.

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.