Merge branch 'debug_interactions' into 'master'

configure.ac

examples/check_interactions.sh

examples/check_ngbs.py

src/debug.c

src/hydro.h

src/hydro/DebugInteractions/hydro.h

src/hydro/DebugInteractions/hydro_debug.h

src/hydro/DebugInteractions/hydro_iact.h

src/hydro/DebugInteractions/hydro_io.h

src/hydro/DebugInteractions/hydro_part.h

src/hydro/Gadget2/hydro.h

src/hydro/Gadget2/hydro_iact.h

src/hydro/Gadget2/hydro_io.h

src/hydro/Gadget2/hydro_part.h

src/hydro_io.h

src/part.h

src/runner.c

src/runner_doiact.h

src/runner_doiact_vec.c