`clearance` arg in pseudo packings not working as it should

[jgostick]

I am trying to make a image of single points (r=1) with a clearance of 10, but the pseudo_gravity_packing won't let me. It complains that a negative particle size is not allowed or something like that. I guess the sanity check I put in there was not robust enough.

[jgostick]

There is actually a bug. The clearance is being removed from the particle radius after insertion, instead of being added to the background before insertion.