Merge branch 'improved_grackle' into 'master'

configure.ac

examples/CoolingBox/coolingBox.yml

examples/main.c

examples/parameter_example.yml

src/Makefile.am

src/chemistry.c

src/chemistry.h

src/chemistry/EAGLE/chemistry.h

src/chemistry/EAGLE/chemistry_struct.h

src/chemistry/gear/chemistry.h

src/chemistry/gear/chemistry_io.h

src/chemistry/gear/chemistry_struct.h

src/chemistry/none/chemistry.h

src/chemistry/none/chemistry_struct.h

src/cooling/EAGLE/cooling.h

src/cooling/EAGLE/cooling_io.h

src/cooling/const_du/cooling.h

src/cooling/const_du/cooling_io.h

src/cooling/const_lambda/cooling.h

src/cooling/const_lambda/cooling_io.h