Presentation 3 context lacks definition of "Model" type

[azaroth42]

Likely as there isn't a term in DC Types where we get the other content type values from. Can fix without a new version as the context isn't semantically versioned - we just need a term from an existing (preferably) ontology to map "Model" to.

See also: https://github.com/dcmi/usage/issues/105

[azaroth42]

Propose: https://schema.org/3DModel