Suggestion: Add sunspec version point to common model

[jsbosch]

The existence of a version field as a model point would allow more flexibility for making corrections and other changes to the model specification.

If I understand the philosophy correctly, adding a point at the end of the common model would not break existing implementations. Another option could be to add a model that contains specification metadata, starting with version. Format could be a simple incrementing integer, major.minor, major.minor.patch, etc.

Another option is a per-model version point, but that level of granularity is probably unnecessary.