Rebuild neighborlist only when required:

Keep track of how many atom-pairs are still within rShellSquared and
rebuild neighborlists when this number is too high.

I've also renamed numNeighborPairs to totalComparedNeighborPairs in
neighborlist.h to more accurately describe what it's counting.
Merge pull request #2 from judofyr/soa-adaptive-neighbors

Rebuild neighborlist only when required