N^2 neighbor list

[streeve]

First step for moving all grid-based neighbor lists to Cajita: separate the underlying N^2 list from Verlet list

• Moves some general neighbor concepts to the NeighborList interface file
• VerletListBuilder inherits from N2Builder and only reimplements details for the various levels of parallelism

Verlet parallel: central particle, optional team bins, optional team vector potential neighbors per bin N2 parallel: central particle, optional team potential neighbors