Datatypes are not included in ontology output

[JPEWdev]

The definitions of the datatypes described in the input Markdown are not included in the Ontology output. For example, in the SPDX 3 markdown, a SemVer datatype is defined, but this is not described in the onotology output so it cannot be validated.

[zvr]

I know, @JPEWdev and I had mentioned it in #99 which provided the first pass of RDF generation.

Thanks for opening this issue to help track the implementation.