Cleanup & Encapsulate the COST_VECT class

The COST_VECT class is used in various places by the optimizer for cost calculations. This blueprint involves moving the COST_VECT class to the optimizer namespace and making the members and methods of the class adhere to the various coding standards we enforce in drizzle:

http://drizzle.org/wiki/Coding_Standards

Another part of this task is to make all data members in the class private. Once these tasks are completed, the places where the COST_VECT class is used will then need to be updated. The class is only used in a few places so only a few modifications should be necessary for this.