Merge branch 'gizmo_new' into 'master'

.gitignore

examples/Gradients/gradientsCartesian.yml

examples/Gradients/gradientsRandom.yml

examples/Gradients/gradientsStretched.yml

examples/Gradients/makeICs.py

examples/Gradients/plot.py

examples/Gradients/run.sh

src/adiabatic_index.h

src/cell.c

src/cell.h

src/common_io.c

src/const.h

src/debug.c

src/dimension.h

src/drift.h

src/engine.c

src/hydro.h

src/hydro/Default/hydro.h

src/hydro/Default/hydro_part.h

src/hydro/Gadget2/hydro.h