[IMP-dev] ListRestraint

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.