Include test particle calculations in Ph4

[ErwanH29]

Modified a couple lines on interface.cc and src/idata.cc to optimise calculations including test particles. Collision detection ignores test particles colliding with one another.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 28 days if no further activity occurs. Thank you for your contributions.

[rieder]

I restored the original formatting, so that the review is clearer. I think the bracket I mentioned earlier is in the wrong place, otherwise looks ok.

[rieder]

If you want to update the formatting afterwards (might be good for readability), please make that a separate PR.