[IMP-dev] get_interacting_particles patch

Here is a new patch which includes the get_interacting_particles code. It is not implemented for NonbondedRestraint as that did not appear to be currently needed and required a few changes to the nonbonded lists and so was not entirely trivial.

The other changes from the last version of the patch are:

- TripletChainRestraint now only has one chain (as with PairChainRestraint)

- There is a new PairScore RefineHighPairScore which refines high scoring pairs for use in hierarchical collision detection.