Expose nearest neighbors rsq array at the python level

[Eric Harper]

the rsq array is available to expose at the python level, but for some reason it never was

[Eric Harper]

This looks a bit more complicated than I originally thought...looks like some things may be broken, or became broken at some point in time.

[Eric Harper]

Dual mode operation might be nice e.g. either let freud expand the search radius or give freud a cutoff beyond which it doesn't expand and then it returns a padded array (either -1 or UINTMAX, not sure which it better).

[Eric Harper]

I need someone to test and verify that the new neighbors(i) actually works and doesn't seg fault...since it's entirely possible garbage collection will nuke that list...

An easy fix would be to move the constructor of the pass-back list to the class constructor or move back to making a copy at the python level