constrain.sphere now instantiates a ConstraintSphereGPU in gpu runs
constrain.sphere now instantiates a ConstraintSphereGPU in gpu runs

git-svn-id: 9672dfcc0771119c6516a56fa23977343927474e@3157 fa922fa7-2fde-0310-acd8-f43f465a7996

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue