Cleaned-up more headers. Code compiles faster

examples/Makefile.am

examples/main.c

src/approx_math.h

src/cell.h

src/common_io.c

src/common_io.h

src/debug.c

src/debug.h

src/engine.c

src/engine.h

src/hydro/Default/hydro_iact.h

src/hydro/Gadget2/hydro_iact.h

src/hydro/Minimal/hydro_iact.h

src/lock.h

src/parallel_io.c

src/parallel_io.h

src/part.h

src/runner.c

src/runner.h

src/runner_doiact.h