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'.

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.