[IMP-dev] Nonbonded lists

Does it makes sense to have sphere and non-sphere versions of nonbonded lists (ones with and without non-zero radii)? I know there are contexts where you can get away with the latter, but it seems like we mostly want spheres and the added confusion and maintenance burden might overwhelm the advantage. Thoughts? I don't see myself using the non-sphere version.

If the radii are zero, the difference comes down to fetching a zero attribute and adding it to each radius. And making sure that each particle has such an attribute.