This changeset:
 * Changes name of COST_VECT to Cost_estimate
 * Encapsulates variables
 * Merges io_count and io_avg_cost into one variable: io_cost
 * Modifies code that used COST_VECT to use Cost_estimate instead
 * Adds unit tests for the Cost_estimate class
      
This changeset does NOT change any cost calculations. It is 
purely a refactoring to an encapsulated C++ class following
our coding guidelines.