Applied the formatting style to all files.

.clang-format

examples/main.c

format.sh

src/cell.c

src/common_io.h

src/cycle.h

src/debug.c

src/drift.h

src/engine.c

src/engine.h

src/gravity/Default/gravity.h

src/gravity/Default/gravity_debug.h

src/hydro.h

src/hydro/Default/hydro.h

src/hydro/Default/hydro_debug.h

src/hydro/Default/hydro_iact.h

src/hydro/Gadget2/hydro.h

src/hydro/Gadget2/hydro_debug.h

src/hydro/Gadget2/hydro_io.h

src/hydro/Gizmo/hydro.h