[IMP-dev] Rewrite of connectivityrestraint

I have a rewritten connectivity restraint. The key differences are that - it now looks like the other new restraints (it uses a PairScore and Particles) - the equivalence sets are described purely through the structure of the restraint, rather than, as before, through an attribute. This makes the code simpler and does not remove any documented functionality. - it is actually documented what it does - the test code is simpler

I wanted to get people's comments on the new interface and all.