Orbits work!

I previously discovered that energy was not being conserved. I thought that this was because the integration technique was not working appropriately. But the euler cromer method does a good job of conserving energy with small error. The problem was that at each time step the forces on any given particle were not being reset to zero. So the magnitude of the force on a particle would be continually increasing because at each time step the forces are calculated and added to the force attribute of the particle. So when the particle is far away from any other it continued to be subjected to forces that it experienced when it was originally closer to another particle. For this reason the particle's kinetic energy was never conserved and as a result bounded orbits were impossible.To fix this it was necessary to simply set the force attribute to zero at the beginning of each time step. This is currently done at the beginning of the Gravitation Law execute method but it ought it be done elsewhere because it is unique to that law.