as there shouldn't be any.

Prevent users from generating gas from the ICs if they are running without the hydrodynamics policy.

This reverts commit ccf5f553.

Add the strict-prototype flag and correct all the prototypes as this creates issues with the Cray compiler.

Add a function to check at start-up that the mass content of the simulation matches the given cosmology.

move clearing a recycled cell's sort buffers to when the cell is re-used. the memory is kept a bit longer, but this avoids calling free concurrently in a tight loop.

replace the particle sort by a particle shuffle. this is possible because we're sorting wrt bounded integer indices. this is also more efficient because of the relatively small number of out-of-place particles expected. not parallel though.

Unify the first initialisation of the part and xpart arrays. Correctly set the ti_drift and ti_kick for gparts and sparts.

Better function name for the first initialiation of all the parts at the very start of engine_init_particles()

Add the possibility to read basic chemistry information from the parameter file and propagate this onto the particle at initialisation time. More uniform naming convention for the first initialisation of properties.

Better naming convention for the cell_split variables in the YAML file. New variable for the splitting of gravity pairs.

Separate the time-step information for hydro and gravity tasks into two different set of variables for each cell.

Use the threadpool to collect data over the top-level cells at the end of the timestep. Also only loop over the cells that have tasks in their hierarchy.