Maintain GPS Watch branch for Associations section of Basic xtUML Modeling course

[johnrwolfe]

This issue and the associated branch will remain open indefinitely so that we can provide the partially complete class model and descriptions as a solution for the Associations exercise in the Basic xtUML Modeling course. This branch should NOT be merged into any other branches because it is missing key aspects of the model. Updates to the mainline of the GPS Watch model should be merged into this branch when necessary (i.e., when those updates affect the class model, including the descriptions.).