Implement an optional minimal smoothing length based on the softening of the particle

examples/EAGLE_100/eagle_100.yml

examples/EAGLE_12/eagle_12.yml

examples/EAGLE_25/eagle_25.yml

examples/EAGLE_50/eagle_50.yml

examples/EAGLE_6/eagle_6.yml

examples/IsolatedGalaxy_starformation/isolated_galaxy.yml

examples/SantaBarbara/santa_barbara.yml

examples/SantaBarbara_low/santa_barbara.yml

examples/SmallCosmoVolume/small_cosmo_volume.yml

...ples/SmallCosmoVolume_VELOCIraptor/small_cosmo_volume.yml

examples/SmallCosmoVolume_cooling/small_cosmo_volume.yml

examples/ZeldovichPancake_3D/zeldovichPancake.yml

examples/parameter_example.yml

src/cell.c

src/engine.c

src/engine.h

src/gravity/Default/gravity.h

src/gravity/Potential/gravity.h

src/gravity_properties.c

src/gravity_properties.h