Add cooling_box example and add required functions

.gitignore

examples/cooling_box/CloudyData_UVB=HM2012.h5

examples/cooling_box/cooling.yml

examples/cooling_box/generate_grackle_data.py

examples/cooling_box/getCoolingTable.sh

examples/cooling_box/plot_cooling.py

examples/cooling_box/run.sh

examples/cooling_rate/cooling.yml

examples/cooling_rate/generate_grackle_data.py

examples/cooling_rate/plot_cooling.py

pyswiftsim/structure.py

setup.py

src/cooling_wrapper.c

src/cooling_wrapper.h

src/pyswiftsim_tools.c

src/pyswiftsim_tools.h

src/wrapper.c