Add typing to json-ld

[Panaetius]

Currently, our json-ld output is untyped for most values, with the values just being string literals. It'd be nice to have type annotations for the values to make the KG less ambiguous, to allow SHACL validation based on types and to support python type annotations once we get rid of python 3.5.

For example, currently we have:

"created": "2018-11-23T14:41:57.822Z"

on projects and dataset metadata. Ideally, it should be:

"created":  {
  "@type": "xsd:dateTime", 
  "@value":"2018-11-23T14:41:57.822Z"
}

using, XMLSchema types (with xsd: http://www.w3.org/2001/XMLSchema#).

The same goes for adding xsd:integer, xsd:boolean, xsd:float and so on.

Related to #700 (comment)

[Panaetius]

This is handled by calamus