added source terms

examples/Feedback/feedback.pro

examples/Feedback/feedback.yml

examples/Feedback/getGlass.sh

examples/Feedback/log

examples/Feedback/makeIC.py

examples/main.c

src/Makefile.am

src/cell.h

src/const.h

src/engine.c

src/engine.h

src/equation_of_state.h

src/hydro/Gadget2/hydro.h

src/runner.c

src/sourceterms.c

src/sourceterms.h

src/sourceterms/Default/sourceterms.h

src/sourceterms/Default/sourceterms_iact.h

src/sourceterms/sn_feedback/sn_feedback.c

src/sourceterms/sn_feedback/sn_feedback.h