Split the hydro and gravity kernels into separate files

src/Makefile.am

src/common_io.c

src/common_io.h

src/gravity/Default/gravity_iact.h

src/hydro/Default/hydro_iact.h

src/hydro/Gadget2/hydro_iact.h

src/hydro/Minimal/hydro_iact.h

src/kernel_gravity.c

src/kernel_gravity.h

src/kernel_hydro.c

src/kernel_hydro.h

src/multipole.h

src/scheduler.c

src/space.c