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.

Defer to future 2.0 release

This would be really convenient to do, but should be in a major release.