made sourceterms more general

examples/DiscPatch/HydroStatic/dynamic.pro

examples/DiscPatch/HydroStatic/makeIC.py

src/const.h

src/engine.h

src/runner.c

src/sourceterms.c

src/sourceterms.h

src/sourceterms/Default/sourceterms.h

src/sourceterms/sn_feedback/sn_feedback.h

src/timestep.h