This patch adds a non-bonded list. The implementation is dumb, but
functional. The code more or less follows what was discussed earlier
on the list.
There is also a NonbondedRestraint which applies a PairScore to each
nonbonded pair. There are PairScores for distance between the centers
of the two particles and distance between the two spheres.
The DistanceRestraint now uses the DistancePairScore internally to
remove redundant code.