Changeset 2110 for coopr.pysp/trunk/coopr/pysp/util

Promoted the scenario tree model to a pysp/util package (with Bill's excellent guidance), and eliminated the replicated ScenarioStructure?.py files from the examples directory.

The only place the master scenario tree model exists is now in coopr/pysp/util/scenariomodels.py, and the (abstract) model is called scenario_tree_model.