Spec needs a formal "Definitions" section

"Data Type" and "Property" are described in [Notational Conventions](https://github.com/FamilySearch/gedcomx/blob/master/specifications/conceptual-model-specification.md#12-notational-conventions). 

1. This isn't really good enough, because the two terms are used often in other parts of the specification. Their meanings should be formally defined.

2. "Data Type" is described as a structure with an identifier, and extension, and a list of properties. Unfortunately, the term is used in property tables where possible values include literals such as "string" which don't match that description. There should be separate terms for the two uses, both formally defined.

3. Other terms needing formal definitions: "Vocabulary", "Controlled Vocabulary", and "Foreign Vocabulary".