Added new method for calculating three-particle forces
Half-way through implementing non-periodic systems
Moved own cell into list of neighbors
Changed to new force calculation method and added tests to verify

New force calculation method added where atoms in neighbor cells are
always included in two-particle force loop and three-particle forces are
calculated from lists on each atom.

ThreeParticleForceSystem test added to verify the new method.
Removed commented method to remove atoms in other cells. Added test to
check force sum for larger systems.
Removed output from tests and added back old tests (fails with MPI)
Fixed all tests to ensure compatibility with MPI
Added back extra communicateAtoms step to ensure proper data transfer in
MPI