[IMP-dev] Nonbonded list

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.

There is some trivial testing.