Add cooling_rate scripts, start implementing stellar physics

docs/RST/examples/cooling.rst

examples/cooling_box/cooling.yml

examples/cooling_box/plot_cooling.py

examples/cooling_rate/cooling.yml

examples/cooling_rate/plot_cooling.py

examples/cooling_rate/run.sh

scripts/pyswift_cooling_rate

setup.py

src/cooling_wrapper.c

src/cooling_wrapper.h

src/libstellar.c

src/pyswiftsim_tools.h

src/stellar_evolution_wrapper.c

src/stellar_evolution_wrapper.h