Implement RockSample + fix bugs

- RockSample implementation
- For non-stochastic models, compute the upper bound action as well, and return
  it as the default action for the lower bound (since we CAN actually attain
  the upper bound value).
- Fix bug where fringe upper bound values were being set to 0 whereas fringe
  lower bound values were being computed by the model. Now both are requested
  from the model.