More const changes for use of struct swift_params

src/chemistry.c

src/chemistry.h

src/chemistry/EAGLE/chemistry.h

src/chemistry/GEAR/chemistry.h

src/chemistry/none/chemistry.h

src/cooling.c

src/cooling.h

src/cooling/EAGLE/cooling.h

src/cooling/const_du/cooling.h

src/cooling/const_lambda/cooling.h

src/cooling/grackle/cooling.h

src/cooling/grackle/cooling_io.h

src/cooling/none/cooling.h

src/equation_of_state/ideal_gas/equation_of_state.h

src/equation_of_state/isothermal/equation_of_state.h

src/equation_of_state/planetary/equation_of_state.h

src/parallel_io.c

src/potential.c

src/potential.h

src/potential/disc_patch/potential.h