[IMP-dev] Various proposed changes

- For the Domino optimizer, the right addition on restraints may be to have them return a list of pairs of interacting particles. This way we don't have to worry about the granularity of the restraint and nonbonded lists can just return their current list of nonbonded pairs.

- The ChainPair restraint could be simplified to only contain one chain now that we have reference counting on the PairScores (so you can share pairscores via shared objects rather than having one restraint). This simplifies the restraint code a bit.

- I think ObjectPointer should be made part of the API now that we have reference counting and renamed to IMP::Pointer and it is pretty stable.