Code formatting

src/cooling/const_lambda/cooling.h

src/engine.c

src/potential.c

src/potential.h

src/potential/disc_patch/potential.h

src/potential/isothermal/potential.h

src/potential/none/potential.h

src/potential/point_mass/potential.h

src/potential/softened_isothermal/potential.h

src/space.c

src/statistics.c

src/units.h