Weighted particle filter

- Instead of using ratio of particles, use particle weights everywhere
- Use sequential importance resampling as given in http://en.wikipedia.org/wiki/Particle_filter#Sequential_importance_resampling_.28SIR.29,
  with low variance resampling in the resampling step
- Remove obsDist, a remnant from hierarchical observations. Change some other
  stuff to completely remove support for ObsTree.
- Remove ObsSetNode and link ObsSet directly with child beliefs
- Move computation of best weighted uncertainty observation into
  ObsNodeCollection