Allow country to be either a ISO 3166-1 alpha-2 country code string or a resource

[jmcanterafonseca-iota]

In the current version of the Vocabulary properties that refer to a country are defined as a resource. It would be convenient to allow properties that refer to a country that also admit just a string conformant with the ISO 3166-1 alpha-2 country code

For instance:

{
  "type": "Consignment",
  "destinationCountry": "ES"
}