Particle:
-Added a field for the radius of the particle. This is needed in the
collision calculation.

ParticlePhysics3D:
-Added togglable gravity, spheres (singularity), and collision between
particles
-Added check box toggles for the above change

PhysicalSphere:
-Added the capability to detach for the BranchGroup. This allows us to
delete the spheres that already exist in the universe.