Merge branch 'skip_relink' into 'master'

Only relink parts and gparts when we actually have parts and gparts

Any reason not do this ? We spend some time linking things even when we don't have anything to link. That would speed up `space_rebuild()` and `cell_split()` in the cases without gravity, the latter of those to two being a scalar part of the code this could be quite important.

See merge request !234