Added an (expensive) debugging check to verify that all cells have been drifted to the current point in time before a rebuild or a repartition.

Expunge external_gravity from cell.h

first try to merge in the scheduler_active stuff from the scheduler_skip branch. currently broken, apparently engine_collect_timestep isn't doing anything.

Moved the un-skipping of tasks to a separate function. Abort drifting when forcerbuild is on. Access forcerebuild atomically.

Collect the radiated energy of all particles when computing statistics and output the result in the log file.

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'.