Upgraded neighbour finding matching the COLIBRE fork

src/cell.h

src/cell_convert_part.c

src/cell_drift.c

src/cell_unskip.c

src/runner_doiact_functions_hydro.h

src/runner_doiact_functions_limiter.h

src/runner_doiact_functions_stars.h

src/runner_doiact_hydro.h

src/runner_doiact_limiter.h

src/runner_doiact_stars.h

src/runner_ghost.c

src/runner_main.c