Document reasoning for space invaders in target assoc center finding

The following discussion from !25 (merged) should be addressed:

@inducer started a discussion: (+1 comment)
Just to let you know: there's a difference in our target associattion code with the algorithm from the paper: it assigns centers to all targets for which it finds a close center, as opposed to only targets marked close to sources.

Two questions:
- Why? (Not that I've got a strong reason against, just curiouus.)
- Algorithmically, again just area querying around centers with the disk radius seems simpler, why have you chosen this approach?