I implemented a 1 particle analog of the new Nonbonded restraint. It
stores a list of particles and applies a SingletonScore to each
particle in the list. One SingletonScore is provided which applies a
unary function to the distance to a fixed point. Other ones we might
want are ones which restraint based on a single coordinate or pairs of
coordinates.
On a separate note, I would move that we remove SphericalRestraint
immediately and DistanceRestraint eventually as both roles are better
served by bulk restraints.