First initalisation of particles now happens in space_init() and not in engine_init_particles() any more.

Add entropy to the energy diagnostic file. Use an MPI_Reduce instead of MPI_Allreduce to get the values

Renamed 'h' in cell struct to 'width' and renamed 'h' and 'ih' in the space struct to 'width' and 'iwidth'.

engine_step now writes to a file called timesteps.txt that contains timestep information along with a header.

the super cell is either a non-empty leaf or the first cell for which an interaction task is defined. need to add a condition for gravit tasks once these have been settled.

add a third communication step which exchanges the particles after the kick, just to make sure we have the correct ti_end_min values for each foreign cell.

link the send/recv tasks to all their cells and sub-cells. first step in linking send/recv tasks to density/force tasks.