Grackle chemistry --> GEAR chemistry. Moved the output of subgrid model to a separate part of the HDF5 file.

Removed time step from flux computation. Removed (untested) flux limiter and unused variables in hydro_part. Discovered and fixed a possible bug in Gizmo particle velocities treatment in flux computation.

When compiling with the iosthermal equation of state, read the constant thermal energy from the YAML file.

Implement simple thread barriers to be used on systems where the POSIX implementation does not provide them. We default to the POSIX ones when detected at configure time.

Partially restored old time step criterion. Isolated flux limiter into a separate file and made it configurable. Removed some superfluous gravity variables.

Implemented the changes in ls that prevent gizmo crashes in cases where the mesh is seriously distorted

Add a separate timer for subset sub-cell tasks. Write the timers to a file. Make sure we can compile without timers. Add a configure option to switch the timers on.

Moved the sorting entry structure to a separate file following @jwillis' work in the vectorisation branch.

Added missing SHADOWFAX_SPH and GIZMO_SPH files to src/Makefile.am. Removed unused counter from riemann_solve_brent().

Uses '1' as we could possibly have a second or third requirement for collections...

Replaced global variables by a new hydro_space struct that is part of space. This struct is then passed on to hydro_init_part, and used to initialize the Voronoi cells. For non SHADOWFAX_SPH schemes, this struct is empty. Replaced random_uniform declarations in some unit tests with the tools.h version. Added some more comments to 3D Voronoi code; it is now almost completely documented (meaning I almost completely understand it now).

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