C2MS12 — Consolidate Navigation Data Messages

The various Navigation Data Messages are nearly identical. They should be consolidated into a hierarchy that defines a Navigation Data Message base class and then let the others derive from it for better model structure. The Tracking Data Message has a few extra fields. Otherwise, everything is identical across all messages except for some type values.

I thought about doing this when I was updating the model in 1.1, but would have also had to update the chapter text and didn't have time for all that in 1.1.