Give user option to dump task levels at a given time step frequency

This merge request adds the option to call the scheduler_write_task_level function multiple times during a run and to generate individual files, defined by the current time step number. The default behaviour remains unchanged: When called, as it is in the current master, the resulting file will be task_level.txt. When passed on an integer time step (currently not done anywhere), it will be task_level_XXXX.txt.