Merge branch 'fof_io_and_compile_without_fof' into 'master'

configure.ac

examples/Makefile.am

examples/main.c

src/Makefile.am

src/common_io.c

src/engine.c

src/fof.c

src/fof_io.h

src/fof_struct.h

src/gravity/Default/gravity_io.h

src/gravity/Default/gravity_part.h

src/gravity/Potential/gravity_part.h

src/hydro/Gadget2/hydro_io.h

src/parallel_io.c

src/runner.c

src/serial_io.c

src/single_io.c