When making a task 'none' in scheduler_split, make both t->ci and t->cj NULL to help with the debugging.

Made the check for unlock overflows active even when not running with debugging checks. Also updated the error message to give some guidance about what to do in these situations.

Fixed typo in the hydro-only task splitting criterion. Need to consider both ci and cj's part counts.

Move the activation of FOF tasks to a separate function. Make a separate function call for the splitting of FOF tasks. Do not reset the unlocks or weights of tasks in engine_make_fof_tasks(); they are not used for FOF tasks and mess up the rest of the tasks.

Added communication functions to propagate the new number of star particles after a SF event has occured.

Added basic support for black hole particles. Particles can be read in, attached to gpart, integrated forward in time, drifted and written to snapshots.

Allow the creation of hydro self-tasks in a cell that has only stars as they will be needed to set the super pointer and the corresponding drift/feedback tasks.

Added a force-communication with its dependency on the drift to make sure the data is correctly drifted before being received in the star loop.