Revert much of last commit, need to unskip tasks of progeny as well, note this means that we cannot just skip non-local cells, we just contract to not actually drift those

All active cells must be drifted so do not skip them if rebuilding.

If rebuilding no need to unskip tasks, that will be after the rebuild

Make sure we visit all related sends of the tasks, not just of the cell

This reverts commit d5191f16.

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

Add debugging checks to make sure all the active tasks are scheduled and that no inactive task is being run.

Re-factored the whole external potential mechanism to match what is done for cooling. Renamed 'disk' to 'disc'.

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

Replaced all calls to fminf and fmaxf by the macros that was added for the integer min/max functions.