Moved the time-step limiter interaction functions to one single common location.

src/Makefile.am

src/hydro/AnarchyDU/hydro_iact.h

src/hydro/AnarchyPU/hydro_iact.h

src/hydro/Default/hydro_iact.h

src/hydro/Gadget2/hydro_iact.h

src/hydro/GizmoMFM/hydro_iact.h

src/hydro/GizmoMFV/hydro_iact.h

src/hydro/Minimal/hydro_iact.h

src/hydro/Planetary/hydro_iact.h

src/hydro/PressureEnergy/hydro_iact.h

src/hydro/PressureEnergyMorrisMonaghanAV/hydro_iact.h

src/hydro/PressureEntropy/hydro_iact.h

src/hydro/Shadowswift/hydro_iact.h

src/runner_doiact_limiter.c

src/timestep_limiter_iact.h