Start rewriting logger

examples/main.c

src/engine.c

src/engine.h

src/hydro/Gadget2/hydro.h

src/hydro/Gadget2/hydro_io.h

src/hydro/Gadget2/hydro_part.h

src/logger.c

src/logger.h

src/logger_io.h

src/logger_struct.h

src/single_io.c

src/single_io.h