Completely remove the source term task.

src/engine.c

src/engine.h

src/engine_maketasks.c

src/runner.c

src/sourceterms.c

src/sourceterms.h

src/sourceterms/sn_feedback/sn_feedback.h

src/sourceterms/sn_feedback/sn_feedback_struct.h

src/sourceterms_struct.h

src/task.c

src/task.h

src/timers.c

src/timers.h

tools/task_plots/analyse_tasks.py

tools/task_plots/plot_tasks.py