Common Interface for Neighbor Sources

[Matthew Spellings]

Some freud modules might like to be able to easily swap out one brand of neighbor list generation (LinkCell/NearestNeighbor) for another. It might be handy (and save some work in the future/currently for the spherical harmonic order parameters in particular) to abstract out the process of iterating over particles and their neighbors.

[Matthew Spellings]

I've made a proposal for this at this link. This is relevant (at least partially) to #43, #44, #57, #107, and #110.

[Vyas Ramasubramani]

See Pull Request #99