When not explicitly given checks for GSL are made anyway, when explicitly given the checks must be successful

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

This reverts commit d6acb7a1.

Meant to add to another branch...

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().