Added 4-particle vecangle potential; untested, and configuration.cpp still needs to build the potential dictionary using type,filename tuples as keys

Changed BondAngle to be the product of the energy of potentials applied to the central bond and two angles formed by four particles

Moved ForceEnergy into useful.h, added atomicAdd for ForceEnergy

Commented unwanted conditional in ComputeKernel that made simulations using the softcore code include self-interactions