Merge branch 'update_cooling_grackle' into 'master'

configure.ac

examples/CoolingBox/coolingBox.yml

examples/CoolingBox/getCoolingTable.sh

examples/CoolingBox/run.sh

examples/Makefile.am

examples/parameter_example.yml

src/Makefile.am

src/cooling/grackle/cooling.h

src/cooling/grackle/cooling_struct.h

src/cooling/grackle/grackle_wrapper.c

src/cooling/grackle/grackle_wrapper.h

src/engine.c

src/units.c