Made defineBoundaryNodes() into its own method called once, rather than once per time-step. I defined & removed a specification (_dirList) which described the direction of the boundary node exchange. But it turns out that I think I only need the Direction 1 nodes (the nodes on a certain local spring that are not local), so I cut out _dirList, but left it all commented.

Also made an exchangeSprForc() which sends the spring force calculated on the Direction 1 nodes back to its owner.