Account for zoom region shift in non-periodic runs

In a zoom, the zoom region is shifted to the centre of the box. This will put some particles outside of the box and in non-periodic runs will result in errors.

The first time this is checked is in space_cell_index.c so I've introduced a check to inhibit any particles outside the volume. This isn't an ideal fix but the alternative of wrapping the particles feels like undesirable behaviour if you specifically want to run a non-periodic volume.